NSUbiquitousKeyValueStoreChangedKeysKey datatype fixed

Fixed the datatype declaration for
NSUbiquitousKeyValueStoreChangedKeysKey (it is an NSArray * not an
NSDictionary *)
Added checks for ChangeReasonKey as per Apple docs

See
https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/U
serDefaults/StoringPreferenceDatainiCloud/StoringPreferenceDatainiCloud.
html
Merge pull request #1 from Clafou/master

Compliance with Apple docs