Allow repositories to have more than one (conditional) addons.xml sources

This allows Gotham to be served using the Frodo addons.xml for most addons in addition to gotham-specific add-ons where needed.  The code is from @cptspiff - I just cleaned it up.

We basically just list the most up to date add-on based on version.

Add-on authors contributing an add-on to both repositories will need to take care to make sure that their versions are kept separate for this reason.

@MartijnKaijser please test again - I made a slight change to the repository.