Added group_{add,remove}_user to haas.server and haas.cli.

Conspicuously, there's no group_create yet, so testing this manually is
a bit awkward, but can be done. It's on the TODO list.

Additionally, the functions had been omitted from the docs, so they've
been added there as well.
haas cli now prints out argument names as part of the help.

You now get output like:

    ...
    user_create <username> <password> # comment
    ...

instead of:

    ...
    user_create # comment
    ...

The arguments are pulled from the functions themselves.
Merge remote-tracking branch 'moc/devel' into group_add/remove_user

Conflicts:
	haas/cli.py
	haas/server.py
Merge pull request #131 from zenhack/group_add/remove_user

Group add/remove user
Tweak model for Hnic

- The headnode field is now non-nullable
- The constructor now takes a 'group' argument
Add API calls headnode_{create,delete}_hnic
Add unit tests for headnode_{create,delete}_hnic
Add REST wrappers for headnode_{create,delete}_hnic
Add CLI wrappers for headnode_{create,delete}_hnic
Merge remote-tracking branch 'moc/devel' into cli-argnames

Conflicts:
	haas/cli.py

In addition to fixing the conflicts, I updated some of the docstrings
for the new functions to better mesh with the help output.
Merge pull request #130 from zenhack/cli-argnames

haas cli now prints out argument names as part of the help.
Merge pull request #136 from gsilvis/api-headnode-create-delete-hnic

Api headnode create delete hnic