new mac osx icon

check why/preview: http://dl.dropbox.com/u/7383846/new_bitcoin_qt_mac_icon.png

- according to official bitcoin logo
- no chopped shadow
- retina-screen support

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
added new mac icon to assets-attribution.txt

license: MIT
Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
change new mac icon license to public domain

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge branch 'master' of git://github.com/bitcoin/bitcoin
new mac osx icon (including SVG Version)

- added new created and documented svg version of shaded icon
- changed "B" background to white (no longer transparent)
- removed PSD (Adobe Photoshop) document
- license is now MIT

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2444 from jonasschnelli/master

new mac osx icon