10.6 support, notification center, back/forward improvements

* Updated the base SDK and deployment target so 10.6+ works (should help with issue #4)
    * Modified weak references to unsafe_unretained since 10.6 has no concept of auto-nil weak ref. I haven't gone through to make sure that the code doesn't do something nasty here now that it doesn't get a free nil
    * Removed auto layout and switched the xibs to springs & struts (auto layout is not available in 10.6)

* Added notification center support in 10.8 to show current song

* When switching to previous or next song it now resets the song to the beginning

* If you hit back when a song is more than 5 seconds in it will now go back to the beginning of that song rather than go the previous track. This mimics iTunes behavior.