feat(ngModelOptions): Model update behavior can now be customized

By default, any change on the content will trigger an immediate model update
and form validation. This PR implements a new directive `ng-model-options`
that allow you to override this default behavior in several ways.

You should specify an object with the different parameters.

For example, it allows to trigger an update only when a particular event or
list of events is received by the input using the `updateOn` key. Should you
need multiple events, just assign an array to it.

I.e. `ng-model-options="{ updateOn: 'blur' }"` will update and validate only
after the control loses focus.

If you want to keep the default behavior and just add new events that may
trigger the model update and validation, add "default" as one of the specified
events.

I.e. `ng-model-options="{ updateOn: ['default','submit'] }"`

Also, with the `debounce` option, `ng-model-options` will allow deferring the
actual model update until a timer expires. The timer will be reset each time
an event is triggered.

I.e. `ng-model-options="{ debounce: 500 }" for 500ms after the latest event.

Custom timeouts for each event can be set for each event if you use an object
in `debounce`. This can be useful to force immediate updates on some specific
circumstances (like blur events).

I.e. `ng-model-options="{ updateOn: ['default', 'blur'], debounce: { default: 500, blur: 0} }"`

You can use the directive in any tag so its contents became the default settings
for any child control, although they can be overridden.

Closes angular#1285, angular#2129

docs/content/guide/forms.ngdoc

@@ -181,6 +181,83 @@ This allows us to extend the above example with these features:
 
 
 
+# Custom triggers
+
+By default, any change to the content will trigger a model update and form validation. You can
+override this behavior using the {@link ng.directive:ngModelOptions ngModelOptions} directive to
+bind only to specified list of events. I.e. `ng-model-options="{ updateOn: "blur" }"` will update
+and validate only after the control loses focus. You can set a single event using an array instead
+of a string. I.e. `ng-model-options="{ updateOn: ["mousedown", "blur"] }"`
+
+If you want to keep the default behavior and just add new events that may trigger the model update
+and validation, add "default" as one of the specified events.
+
+I.e. `ng-model-options="{ updateOn: ["default", "blur"] }"`
+
+The following example shows how to override immediate updates. Changes on the inputs within the form will update the model 
+only when the control loses focus (blur event).
+
+<example>
+  <file name="index.html">
+    <div ng-controller="ControllerUpdateOn">
+      <form>
+        Name:
+        <input type="text" ng-model="user.name" ng-model-options="{ updateOn: "blur" }" /><br />
+        Other data:
+        <input type="text" ng-model="user.data" /><br />
+      </form>
+      <pre>username = "{{user.name}}"</pre>
+    </div>
+  </file>
+  <file name="script.js">
+    function ControllerUpdateOn($scope) {
+      $scope.user = {};
+    }
+  </file>
+</example>
+
+
+
+# Non-immediate (debounced) model updates
+
+You can delay the model update/validation by using the `debounce` key with the
+{@link ng.directive:ngModelOptions ngModelOptions} directive. This delay will also apply to
+parsers, validators and model flags like `$dirty` or `$pristine`.
+
+
+I.e. `ng-model-options="{ debounce: 500 }"` will wait for half a second since
+the last content change before triggering the model update and form validation.
+
+If custom triggers are used, custom debouncing timeouts can be set for each event using an object
+in `debounce`. This can be useful to force immediate updates on some specific circumstances
+(like blur events).
+
+I.e. `ng-model-options="{ updateOn: ["default", "blur"], debounce: { default: 500, blur: 0 } }"`
+
+If those attributes are added to an element, they will be applied to all the child elements and controls that inherit from it unless they are
+overridden.
+
+This example shows how to debounce model changes. Model will be updated only 250 milliseconds after last change.
+
+<example>
+  <file name="index.html">
+    <div ng-controller="ControllerUpdateOn">
+      <form>
+        Name:
+        <input type="text" ng-model="user.name" ng-model-options="{ debounce: 250 }" /><br />
+      </form>
+      <pre>username = "{{user.name}}"</pre>
+    </div>
+  </file>
+  <file name="script.js">
+    function ControllerUpdateOn($scope) {
+      $scope.user = {};
+    }
+  </file>
+</example>
+
+
+
 # Custom Validation
 
 Angular provides basic implementation for most common html5 {@link ng.directive:input input}