Updated & simplified build_osx.py for a reliable Mac app without requiring any homebrew dependencies for users

This simplified py2app config allows OS X users to download and run BitMessage without needing to install any homebrew dependencies. Just download and run.

It works reliably for 10.8.4, tested across different machines, and has received good feedback from other users: https://bitmessage.org/forum/index.php/topic,2761.0.html

The issue with the current build_osx.py is that it tends to produce an app that doesn't work reliably across different Macs (even when they're running the same 10.8.4 version). I started from scratch and built up a new py2app config one line at a time until everything worked reliably and still didn't require any dependencies to be installed by the end user. 

I think this is the better one to use because it's greatly simplified and more reliable. Then it can be tweaked as needed in the future. (The current osx.sh produces a very nice DMG, so I didn't change that at all.)

Also, can we get a download link on the homepage to a Mac DMG please? Probably should specify "for OS X 10.8.4+". http://sourceforge.net/projects/bitmessagemac/files/