Rename UNREGISTERED_NICKNAME to DEFAULT_NICKNAME.
Move some features from rfc1459 to pydle.features.account.
Change nickname setting through attribute to Client.set_nickname().
Change Client.mode()/Client.topic() to Client.set_mode()/Client.set_topic().
Also set registration completed on missing MOTD.
Create on_user_online/on_user_offline callbacks for MONITOR extension.
Update documentation on recent changes.
Reorder features.