Add -[RACSignal setKeyPath:onObject:nilValue:]
Refactor RAC() to accept >= 2 arguments and a nil value
Don't send nil for an integer value in tests
Refactor properties and bindings to accept a nilValue
Move some nilValue testing to RACPropertySignalExamples
Test that -setNilValueForKey: still works
observee -> object
Always require two arguments for RACObserve

This is consistent with the updated RAC() and RACBind() macros.
Merge remote-tracking branch 'origin/2.0-development' into default-binding-values
Switch to using an array instead of a dictionary for performance.
Merge pull request #670 from ReactiveCocoa/default-binding-values

Accept a placeholder nilValue for all bindings
Merge remote-tracking branch 'origin/thread-local-stack-counter' into thread-local-stack-counter
Merge branch '2.0-development' into thread-local-stack-counter

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACObservablePropertySubject.m