new mac osx icon

check why/preview: http://dl.dropbox.com/u/7383846/new_bitcoin_qt_mac_icon.png

If pull is accepted, windows, etc. icons will follow.

- according to official bitcoin logo
- no chopped shadow
- retina-screen support

info:
the psd (Adobe Photoshop) file (bitcoin.psd) is for further changes (contains unrendered effects)