[WIP] ReactiveCocoa 3.0

**:warning: NOT READY TO MERGE :warning:**

**See the [CHANGELOG](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/3.0-development/CHANGELOG.md) for a list of all major changes, and information on how to update your project.**

Performance improvements and critical fixes should continue to be applied to RAC 2.x until this branch is merged. After merging, 2.x versions will no longer be actively developed.