Add -[RACSignal deliverOnMainThread]
Queue events and drain them on the main thread.
Merge branch 'master' into deliver-to-ui
Make delivery code more similar to RACSubscriber.
Remove unnecessary nil check.
Revert to delivering one event per run loop.

This reverts commits 97da8702ebbfc6f869ca4a20b255a6825d31cc23, 07c927e976eaf7df76a2c73947d534ebcc8e7d12, ade0a6310bb86121a3142527f54cd74169ec0958.
Fix method brace style
Refactor -deliverOnMainThread to deduplicate code
Bring queueLength into each signal subscription
No need for a barrier upon incrementing

At least, I think this is correct, since we’re not trying to
synchronize with any other state at the same time.

/cc @jlawton
Add unit tests for -deliverOnMainThread
Merge pull request #1618 from ReactiveCocoa/pr/1425

-deliverOnMainThread redux