Merge jspahrsummers/RxSwift@15734da
Update bridging, move into implementation files
Better docs for toSignal
Move Observable.constant definition up a bit
Restore bridging header to improve type safety

fromSignal() is kinda janky and unsafe, so extending RACSignal is a
better idea.
Add Promise.toSignal
Add Action.swift to project and targets
Add Observable.toInfiniteSignal
Add RACSignal.toObservable
Add RACSignal.toPromise
Add RACCommand.toAction
Remove useless `final` annotations in extensions