use NSMutableSet instead of NSMutableArray and refine some codes
changed observer passing
Merge remote-tracking branch 'krzysztofzablocki/master'

* krzysztofzablocki/master:
  changed observer passing

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