Allow nil for -addDisposable:
Remove nil checks before invoking -addDisposable:
Merge pull request #879 from ReactiveCocoa/add-nil-disposable

Allow nil for -[RACCompoundDisposable addDisposable:]
Merge remote-tracking branch 'origin/master' into 3.0-development
Merge branch '3.0-development' into deprecations
Merge branch 'deprecations' into promises