Add methods to resolve groups to the UserService

A customer implementing a FishEyeAuthenticator (see https://answers.atlassian.com/questions/15268/get-list-of-groups-for-user-in-crucible-fisheye-2-7) couldn't find a supported way to resolve a list of groups for a user.

We should add suitable methods for group resolution to the UserService exposed in FishEye's API.