Miscellaneous improvements

Changes:
- Changed minimum version requirement to iOS 7.0. Note that if you use this library with an app whose minimum target is 7.0, you should still gate off any advanced features.
- Added macros for checking iOS version at runtime
- Changed sample app to work with iOS 7 targets
Added result deduping logic to HKWMentionsCreationStateMachine

- Added support for deduping within the results of a response and when appending
  a new response to existing response data

- Modified the asyncRetrieveEntitiesForKeyString completion block to take an
  additional boolean argument indicating if results should be deduped.

- Exposed a (NSString *)uniqueId method to HKWMentionsEntityProtocol which allows
  the caller to uniquely identify a mentions entity that may require more
  information than just the entityId