Added support for both iOS and OSX

First pull request here, so let me know if I'm doing anything wrong.

What I did:

1. Changed the filenames from UIView+FrameAccessor.h/m to FrameAccessor.h/m.

2. Put in an ifdef in the top of FrameAccessor.h to determine wether we are on iOS or OSX and setting the view thereafter (UIView or NSView).

3. Added a podspec to be used publicly. You should probably change the github values to your values.