Add 'Audio' to the categories in the .desktop file

As per
http://blogs.gnome.org/hughsie/2013/09/03/broken-desktop-files/
it should have both 'AudioVideo' and one or both of 'Audio' or 'Video'
in order to be categorized correctly.
Merge pull request #157 from kepstin/desktop-fix

Add 'Audio' to the categories in the .desktop file