ReactiveCocoa 2.0

**See the [CHANGELOG](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/2.0-development/CHANGELOG.md) for a list of all major and breaking changes, and information on how to update your project.**

Performance improvements and critical fixes should continue to be applied to RAC 1.x until this branch is merged. After merging, 1.x versions will no longer be actively developed.

:sparkles: Many thanks to everyone who's been helping! :sparkles: