Implemented basic events.

The Torrent class now has a callback method to which you can assign to.
It's called everytime a change occured in the torrent state, could be useful for
example to send a notification to the user when a torrent finishes.
Merged wip/torrent-event in master
Better events
Move UI settings out of core
Sane size conversion
Fixes #108, #109
Remove useless things
Merge pull request #111 from fuyukaidesu/master

Events.