Use a CFSet instead of NSSet for RACActiveSignals

Also, pointer equality and pointer hashes instead of -isEqual: and
-hash.

Fixes #734.
Merge pull request #735 from ReactiveCocoa/active-signals-optimization

Use a CFSet instead of NSSet for RACActiveSignals
Xcode 5dacted
Merge branch '2.0-development' into uikit-bindings
Style fix.