Add support for translations

Now that the data is getting upstreamed and progressively translated, it's a shame to lose that work and only show English in the installer.

Note: I tested this with one package, but I did not try to regenerate all of Fedora 20 metadata, so I don't know how big it would be.
Note 2: ConfigParser is a bit dumb, and forces all keys to lowercase. It should not be a problem, as the gnome-software side uses case insensitive comparison, but I thought I would mention it anyway.