Merge pull request #12 from minorblend/master

use __AH_WEAK macro instead of __unsafe_unretained
Update SFObservers.h

Recommend allowing multiple registrations by default.  I just saw a case where a UIKit class's private implementation triggered an error due to this.
Merge pull request #14 from jacobjennings/patch-1

Update SFObservers.h
Merge remote-tracking branch 'krzysztofzablocki/master'

* krzysztofzablocki/master:
  Update SFObservers.h