Document major differences from Rx

Includes per-method documentation where names differ, and a new [Differences from Rx](https://github.com/github/ReactiveCocoa/blob/rx-differences/Documentation/DifferencesFromRx.md) document, which tries to cover high-level differences.

Completes #176.

@andrask @cwharris @xpaulbettsx @Haacked @half-ogre @aroben I'd appreciate it if someone more familiar with Rx could fact-check this, or point out anything major I missed. :sparkles: