Added Appstream XML data. Needs to be reviewed
Fixed appstream.xml more fit to project. Changed description coming from 'debian/mixxx.sgml' and changed me from copyright to author also fixed correct email address
Changed 'Bulk BPM and beat detection.' to 'Beat and key detection' what should open to most people.
Changed '&#38; (HTML: &)' to and which I think correct
Every comma now have space after appearence
Merge pull request #389 from illuusio/appstream-fix

Added Appstream XML data for better Gnome and KDE integration.