Download and parse bwauth files

Recently tor added its bandwidth authority files to the DirPort...

https://gitweb.torproject.org/torspec.git/commit/?id=365d371

We should add a parser and support in stem.descriptor.remote. This feature was added in tor 0.3.5 which the dirauths aren't yet running #26694 (moved) will update dir-spec.txt with the Tor version that add this feature once it's merged (#21377 (moved)). Gonna wait to see this working in practice before we add stem support.