Cherry picked from aborted 3.0 - Replace -didSubscribe…: with a disposable property
Charry picked from aborted 3.0 - Add RACLiveSubscriber, reduce subscriber allocations

Since RACLiveSubscriber fulfills the use cases of _both_ RACSubscriber
and RACPassthroughSubscriber, we can avoid creating _two_ subscribers
for every block-based subscription, and just reuse the same object
instead.
Undo changes made to the project file by Xcode - should get folded into 076d38e.
Merge pull request #1597 from talko/backportlivesubscriber

Backport RACLiveSubscriber from aborted 3.0 branch.