Add rac_didDeallocSignal to NSObject

Convinient signal for infinite signal termination.

like this:
  [infiniteSignal takeUntil:[anObject rac_didDeallocSignal]]
rewrite with rac_addDeallocDisposable:
add test for rac_didDeallocSignal
Documentation
Style fixes
Remove unnecessary test (actually testing -takeUntil:)
Merge branch 'did-dealloc-signal' (pull request #376)