Bencode reader

This is a complete rewrite of the bencode decoder since the old one was based on `[]byte` which is bad for multibyte characters since they weren't read correctly.

This new version is using `bufio.Reader` and therefore is _much_ better. And faster.

[![Coverage Status](https://coveralls.io/repos/octorrent/octorrent/badge.png?branch=bencode-reader)](https://coveralls.io/r/octorrent/octorrent?branch=bencode-reader)