Keep track of IRC users

[spyked]

A basic implementation should allow the DiceBot to echo the IRC user name of the person who made a roll (e.g. so that multiple users can do the same roll at once).

Ideally, the bot should be able to separate the game master from players, add/remove players from the game, keep track of player stats and so on. These will be documented in future issues.

[spyked]

Partially solved in 5908ce2. DiceBot answers to users both privately and in the channel and echoes their name.