hook_invite needs to return 'code' on 'escalate' $op operation

The hook_invite provides basic operations for the invite lifecycle (invite, escalate, cancel), but the 'escalate' does no contain the code that was used for the invite. This may be valuable to another module as it can track unique invites and map them for use in lookup.
This should be relatively easy as the $invite variable contains the code to be passed back in the hook in most situations -- see patch.