Create chat history table for subscription based rooms

TODO:
 * Add user command to view history
 * (UNDECIDED) Add option to view n lines from history
Untested chat history for chatrooms

Can be only used by registered users at the moment
Update README and CHANGELOG
Add to history table after the timestamp has been prepended

Tested on server. Works fine.
Merge pull request #6 from hjpotter92/feature/chatroom-history

Chat history feature for subscription based chatrooms