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.
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