Reimplement -catch: without subscribeForever
Add basic tests for -catch:
Test disposal after -catch:
Spell good
Fix -catch: disposable test
Use RACSerialDisposable for -catch: subscription
Assert that -catch: block returns non-nil
Merge pull request #715 from ReactiveCocoa/catchy

Reimplement -catch: without subscribeForever, and add basic tests