Added GET /group/membership support.

Hi,

I've added support for GET /group/membership support to dump the whole membership associations database.
https://developer.atlassian.com/display/CROWDDEV/Crowd+REST+Resources

A bit ugly, but works. ;-)

I've authored a gitolite crowd integration module, I'm going to publish that soon.
This will be an example usage fort this feature.

Regards,
  Attila