Create database models and define how they interact with the protocol handlers

[Kit Barnes]

Shit's going to have to go into a database eventually, lets define the models for it. Plan at the moment is to use peewee as it seems nice.

Also regarding interaction with protocol: I say we make the models use the protocol module as any outside user would: via the hooks. This keeps the protocol bits nice and simple and free of janky model code.

[Kit Barnes]

I've considered putting model code into possel rather than pircel but I think message and state storage is a common enough desire in IRC applications that we should handle it in the library.