Support setting an API URL for the audio scrobbler engine

This patch allows a user to run 'gconftool-2 --set /apps/banshee-1/plugins/audioscrobbler/api_url --type string http://turtle.libre.fm/' for example to have the AudioScrobbler engine scrobble to the new libre.fm service, which implements the last.fm scrobbling web service API.

Proper multiple account support is needed however to have both last.fm and libre.fm work together. Setting this value is for the elite freedom lovers only right now.