[ON HOLD] ReactiveCocoa 3.0

**:warning: This may be supplanted by #1382. Please do not use! :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.

Because migration is hard and time-consuming, _99% of RAC 2.x code will continue to work under RAC 3.0 without any changes_. You'll see deprecation warnings by default, but even these can be temporarily disabled by defining [`WE_PROMISE_TO_MIGRATE_TO_REACTIVECOCOA_3_0`](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/3.0-development/ReactiveCocoaFramework/ReactiveCocoa/RACDeprecated.h) before any framework headers are imported.

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.