Support mysql backend

[erikjohnston]

I have zero idea if this works.

[NegativeMjark]

I've tested it locally using:

./panopticon --port 9001 --db-driver mysql --db "root:aaaaa@tcp/panopticon" &
curl -d '{"daily_active_users": 10, "timestamp": 20, "total_users": 123, "total_room_count": 17, "daily_messages": 9, "uptime_seconds": 19, "homeserver": "many.turtles"}' http://localhost:9001/push

It seems to successfully create the table and write the stats.

[NegativeMjark]

It looks like all the changes needed were the usual mysql annoyances.

I've left the sqlite3 support for now, but it would be reasonably easy to remove it to make the code simpler.