Create a bw auth spec

We should create a spec for the bw authorities. We have proposals 160 and 161, but they are a little old and don't describe exactly how the system works now:

https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/161-computing-bandwidth-adjustments.txt https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/160-bandwidth-offset.txt