RACCommand improvements

**:warning: NOT READY TO MERGE :warning:**

From the ashes of [RACAsyncCommand](https://github.com/ReactiveCocoa/ReactiveCocoa/commit/1e7986eda6c518a5638fb9f16f6f8b3df2aef174) and #302, an asynchronous phoenix arises.

This still needs tests for the new functionality, but the new [RACCommand](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/command-signals-redux/ReactiveCocoaFramework/ReactiveCocoa/RACCommand.h) interface and implementation is ready for review.