Parse networkstatus documents for running flag and vanilla bridges

Previously, rdsys only supported bridges with transports and was not parsing networkstatus files for the Running flags. This adds support for parsing networkstatus files using zoossh. We follow the logic used by BridgeDB, and first parse bridge information from the networkstatus document. We then update each bridge with transports from the extrainfo files.

We only hand out bridges as vanilla bridges if they do not have other transports, and do not distribute bridges if they do not have a Running flag.

Closes #38 (closed)