Use an unsafe self reference in -[RACSubscriber init]

A weak reference is unnecessarily expensive, and this disposable is
guaranteed to be used before the captured object disappears.
Reference disposables directly in -didSubscribe…

Avoids any need for weak references, since the RACSubscriber is no
longer being captured.
Merge pull request #1500 from ReactiveCocoa/unsafe-subscriber-ref

Use an unsafe self reference in -[RACSubscriber init]