Add users susers gusers commands to fix #18

These commands are for listing users at the three different access
levels server, global, and channel.
Clean up code for user lists.

- Add check to store *Users methods to return no error on empty
  database.
- Add spacing to the output of user lists.
- Add tests for user list commands.
- Reorder so guser code comes first.
Merge branch 'userlists'