Remove RACPropertySubject
Refactor RACBinding as endpoints with signals/subscribers
RACObservablePropertySubject -> RACKVOBinding
Move endpoints' logic into RACBinding directly
Refactor NSObject+RACAppKitBindings
Refactor unit tests

@Coneko I think removing all the previous examples for RACBinding is
correct, but can you make sure I'm not missing anything?
Extend RACKVOBinding lifetime as long as the KVO
Fix two-way binding in RACKVOBinding
Merge remote-tracking branch 'origin/2.0-development' into refactor-properties-and-bindings

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa.xcodeproj/project.pbxproj
Remove obsolete comment
Merge remote-tracking branch 'origin/2.0-development' into refactor-properties-and-bindings

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa.xcodeproj/project.pbxproj
Refactor RACBinding interface with endpoints yet again
Refactor RACKVOBinding and RACAppKitBindings
Refactor unit tests
Note that RACBindingEndpoint shouldn't be instantiated
Merge remote-tracking branch 'origin/2.0-development' into refactor-properties-and-bindings
Replace rumors/facts terminology with left/right
Document which bindings start with what values
Add shared examples for all view bindings

Right now, this is just for our AppKit bindings wrapper, but UIKit will
be added in the future, and this might be useful to test other AppKit
classes.
Merge remote-tracking branch 'origin/2.0-development' into refactor-properties-and-bindings