Remove note about mac osx support

There won't be any osx support before qt 5.4 has been released (if the qt webkit does not crash as it is doing with qt 5.3)
Fix a small branch in get_git_branch_name if there are more than 1 branch
Merge branch 'master' into pyqt5_support

Conflicts:
	mellowplayer/system.py
Make mellow player a single instance application

Fix raising windows from the gnome MPRIS2 player extension