[#1987] group_list returns groups of passed type.

group_list get action will return group types specified by passing
the optional parameter type=<group type>. The default is 'group'.
[#1987] PEP8ing before changes
[#1987] IGroupForm plugins can define controller.

If you're using a custom controller with an IGroupForm plugin you can
define it in the plugin with 'group_controller()'. This will be used
when registering the plugin mappings. Defaults to 'group' if not defined
in the plugin.
Merge pull request #1987 from brew/1987-custom-group-changes

Changes to allow better use of custom group types in IGroupForm extensions