small clean-up for civicrm_group_roles.module

We did some work to try and integrate the civicrm_group_roles module with a call to user_save that was removing roles. We quickly found ourselves in and endless loop between user_save and hook_civicrm_post ( civicrm_group_roles_civicrm_post ) when we added an expression for $op= 'update'. we had to add the code at $op = 'update' because that is the only point in editing a contact when the $user['roles'] and $edit['roles'] array passed to user_save are different.

Consequently, we simply added the civicrm_group_roles_add_remove_groups function which can be called immediately after the user_save call with many of the same parameters, it's not ideal but it's better than forcing Drupal coders to sort through 4 API calls, etc...

the patch is against trunck but will apply against 3.3 also.

Also, replaced array_diff with array_diff_key as you cannot reliably depend on the value in the elements of $user->roles as you can the key