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'
Update old packages.
Update README.md

Add code coverage badge.
Add CTCP support to irc package.
Add new CTCP, CTCPChannel and CTCPReply handlers.

- Add CTCPHandler, CTCPChannelHandler, CTCPReplyHandler interfaces for
  event handling.
Add CTCP helpers to Message struct.