Floats are silently rounded to integers

 `Bencdr.decode Bencodr.bencode(3.1415) == 3`

This conversion happens silently, which makes it a bit dangerous, because people will not know that it happens, and with some data this will create issues.
It would be great if not having float support could be added to the documentation. Also, maybe throwing an error instead of silently rounding is better behaviour in cases like this.
