Future proof KVO implementation

As explained in http://www.dribin.org/dave/blog/archives/2008/09/24/proper_kvo_usage/ the only safe way to know if you must call `[super observeValueForKeyPath:ofObject:change:context:]` is to use the context parameter. You also want to pass this context parameter when removing self as an observer.
Minor code style formatting
More minor code style formatting
Remove obsolete comment on `AFNetworkingTaskDidStartNotification`
Use the AF prefix for consistency with the rest of the library
Merge pull request #1231 from 0xced/future-proof-kvo

Future proof KVO implementation
Minor refactoring and reformatting of task state KVO

Always dispatch notifications on main
Decrement network status activity indicator on AFNetworkingTaskDidSuspendNotification
Refactor URL session task creation methods to AFURLSessionManager, changing callback block structure to be HTTP agnostic
Removing unnecessary &* for __weak casts
Checking keypath rather than -respondsToSelector: in KVO method
Minor documentation edit
Remove TODO in UIButton category
[Issue #1234] Setting default string encoding
Temporarily removing Travis until tests are ported
Bumping version to 2.0-RC2
Setting default Accept headers for request serializers

Setting default request serializer for AFHTTPClient to JSON