Fixes #279 by providing a new paster command

'paster help group' for more info.

 - Added adduser and removeuser to fix #279
 - show who the admins/users are and a small tidyup
 - make it easy to add/remove datasets to/from groups
Change to DatasetCmd for #279

Now no longer performs an assert and trigger an exception, but does return 1 from the process
Fixes problem with purging groups in #279
Merge branch 'group_paster_command' into release-v1.7.1-dgu