Improved Bot#rename(nick).
Improved Bot#rename(nick).

- Bot#rename(nick) slices the desired nick if it's longer than the network permits.
- Bot#rename(nick) now returns a Celluloid::Future. Example:
    `future = @bot.rename('new_nick')`
    `future.value # might return:`
      - a String with the old nick if no changes would happen.
      - a String with the new nick.
      - :erroneous_nickname if the nickname is erroneous (e.g. 'NickServ' or '!&$foo').
      - :nickname_in_use if the nickname is taken.

tbuehlmann/vetinari