Added -[NSIndexSet mapIndex:] method.
Added doc comment for new method.
Merge pull request #128 from kaelin/master

Add -[NSIndexSet mapIndex:] method, thanks to @kaelin.
Unit test and fix for block observer multiplexing. Closes issue #132.
A2BlockInvocation: Use NSDictionary to retain arguments. More robust string copying. Fixes analyzer issues; closes #125.
Rewrite parts of NSObject+BlockObservation to enable automatic deobservation, courtesy of ReactiveCocoa. Closes #115.
Atomic versions of associated properties. Closes #112.