Added a property to NSControl+RACCommandSupport to allow a secondary signal to influence the control's `enabled`.
Merge branch 'NSControl-mergeEnabledBindingSignals' of https://github.com/woolsweater/ReactiveCocoa into NSControl-mergeEnabledBindingSignals

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/NSControl+RACCommandSupport.h