Added an appdata file so it'll be better represented in gnome-software. 

There aren't any screenshots yet, so I've left them blank.

I've chopped a few lines to make sure the file validates with
appdata-validate.

No changes have been made to the build system. If you could please
modify it such that this file is installed to /usr/share/appdata/ that
would be great. It's optional since I can install it manually in the
Fedora spec too.

More information here:
https://lists.fedoraproject.org/pipermail/devel/2013-September/188799.html
http://people.freedesktop.org/~hughsient/appdata/
http://blogs.gnome.org/hughsie/2013/10/08/how-to-take-169-screenshots/