Add sample for App Engine Datastore setDistinct projection queries.

Also update checkstyle to allow 1-letter variable names, since it
doesn't make sense to give long names to something like "arbitrary
column A that doesn't actually mean anything".

google-checks.xml

@@ -121,13 +121,13 @@
              value="Member name ''{0}'' must match pattern ''{1}''."/>
         </module>
         <module name="ParameterName">
-            <property name="format" value="^[a-z][a-z0-9][a-zA-Z0-9]*$"/>
+            <property name="format" value="^[a-z][a-zA-Z0-9]*$"/>
             <message key="name.invalidPattern"
              value="Parameter name ''{0}'' must match pattern ''{1}''."/>
         </module>
         <module name="LocalVariableName">
             <property name="tokens" value="VARIABLE_DEF"/>
-            <property name="format" value="^[a-z][a-z0-9][a-zA-Z0-9]*$"/>
+            <property name="format" value="^[a-z][a-zA-Z0-9]*$"/>
             <property name="allowOneCharVarInForLoop" value="true"/>
             <message key="name.invalidPattern"
              value="Local variable name ''{0}'' must match pattern ''{1}''."/>