Import 2 functions from unifi-lab: get_users and get_user_groups

Obtained via unifi-lab, adds 2 simple functions to get more
user and configuration data.

get_users likely represents a couple of things that are shown
in the "All Clients" tab of UniFi's web interface. (though not all)

In contrast to get_clients, it shows both curently connected as well
as disconnected clients. Infortunately I don't have knowledge about
how long exactly this data is stored in the DB, but it seems to store
quite some data back in time.

get_user_groups gives mostly QoS rate limitation information
about user groups defined in the web interface.
Merge pull request #7 from matsimon/data-functions

Import 2 functions from unifi-lab: get_users and get_user_groups