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
Merge pull request #670 from ReactiveCocoa/default-binding-values

Accept a placeholder nilValue for all bindings