Add Convos::Archive classes for storing IRC log to disk

I split your original code into two classes @marcusramberg. I think this makes it easier to extend later on.

I also moved the "archive" object to Convos::Core, and a "save" event to Convos::Core::Connection, so we only need one archive object. Not quite sure what is right or not here.

Anyhow, the "save" event in Convos::Core::Connection could be the first step to making plugins for Convos :)