[WIP, RFC] torrent file parser

This is a work in progress, I'm creating this pull request to get some reviews/comments.

Things to do:

- [x] Implement `toBEncode` methods.
- [x] Expose lenses.
- [x] Implement `Arbitrary` instances.
- [x] Add tests. (some example files files https://github.com/Fuuzetsu/torrentinfo/tree/master/test/files)

How should I add tests? Should I add some example torrent files and try to parse them? As for properties, all I can think is `fromBEncode . toBEncode $ d = Right d`. Any other ideas?
@Fuuzetsu I can't see anything about non-emptiness of `files` field:

https://wiki.theory.org/BitTorrentSpecification
http://bittorrent.org/beps/bep_0003.html

I agree that it's not useful but I think it's still valid.