[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?
Actually most of ByteStrings can be converted to Text or String depending on the use but I say let's wait until they're used. Other than md5 sums I don't think they should kept as ByteString.