Consider and add appropriate hooks

[Kit Barnes]

Discussion in here, we'll fill in this wiki page as we go; hopefully resulting in a decent specification of the hooks.

[Kit Barnes]

So at the moment the user can add callbacks to any of the RPL_* or ERR_* commands from the server either by registering them or by subclassing and providing an on_* method (e.g. on_rpl_namreply).

Should we define a few friendlier names? e.g. on_privmsg could be better called on_message perhaps.

We should almost certainly define a few more complicated triggers. e.g. on_channel_message for privmsg to a channel, on_private_message too.