Adds release group urls

[lionelerard]

Adds urls to the release group model.

[volontarian]

I see you geared to Artist#urls so I will merge it.

Later I will fix the following issue which is valid for Artist#urls, too.

Consider this xml:

http://en.wikipedia.org/wiki/Empire_(Kasabian_album)http://de.wikipedia.org/wiki/Empire_(Kasabian_album)

=> { :urls => { :wikipedia => 'http://de.wikipedia.org/wiki/Empire_(Kasabian_album)' } } ... but there are 2 urls.

should return { :urls => { :wikipedia => ['http://en.wikipedia.org/wiki/Empire_(Kasabian_album)', 'http://de.wikipedia.org/wiki/Empire_(Kasabian_album)'] }