Create a module for color utilities

This abstracts the logic done on paymentcategoryeditor and creates a generic
one that anyone can use for getting random colors from tango pallet.
It's possible now to do:

  from stoqlib.lib.colorutils import get_random_color
  color = get_random_color()

Change-Id: Ic63836ce5a53736803d1672bb8f02ebb836842a5
Implement LoginUser.get_active_users

Calling it will return a result set containing just active users.
This works just like the other person facets work. (e.g.  Client.get_active_clients)

Change-Id: I35b65249395a7f15c60570e07cf71734254c2142
Merge changes I35b65249,Ic63836ce

* changes:
  Implement LoginUser.get_active_users
  Create a module for color utilities