[#669] Fix member_list call when given valid group name.

Now returns lists of (id, type, capacity) tuples
where type is a string. Previously, type was a
Python class object which could not always be
serialized to JSON in the API controller.
[#669] PEP8
[#669] Raise NotFound if group is not found in member_list.

Allows a proper error to be generated in the API
controller instead of just giving a 500 error.
[#669] Tidy up imports
Merge branch '669-member-list-broken'