1.0.0-beta2

• added Selector
• removed key*() methods
• renamed argument $field to $key, moved it to end of the method arguments
  and made it optional for all test methods
• removed argument $field from key()
• removed argument $strict from true(), false() and oneOf()
• added atLeast() selector
• added exactly() selector
• added all() selector
• added atMost() selector
• added count() selector
• renamed oneOf() to in()
• added valid()
• added invalid()
• optimized andValid(), andInvalid(), orValid() and orInvalid()