Add the first set of handlers to Store.

- Add the handlers: join, quit, part, nick
- Refactor the UserModes into UserModes and UserModesMeta
- Create UserChannel and ChannelUser in place of ChannelUser which was
  insufficient.
- Amend list of hardcoded constants in irc.
Add implementation for everything but mode handler
Rename modes to channelmodes.

- Rename UserModesMeta to UserModeKinds
- Moved UserModeKinds to modes_common.