fixing retain cycle
Fixing retain cycle
Update SFObservers/NSObject+SFObservers.m
Update SFObservers/NSNotificationCenter+SFObservers.m
- ensure swizzling always swaps the methods of correct class
- fix a crash when the given context points to a non-objc object
Merge pull request #11 from lono175/upstream/master2

Fix a crash when the given context points to a non-objc object
Merge branch 'upstream/master' of github.com:lono175/SFObservers into lono175-upstream/master
Merge branch 'refs/heads/master' into no_swizzling

Conflicts:
	SFObservers/NSNotificationCenter+SFObservers.m
	SFObservers/NSObject+SFObservers.m