Rename bridgedb.Main.startup → bridgedb.Main.run.

 * REMOVE old `bridgedb.Main.run` function, which did nothing but call
   `bridgedb.Main.startup` and act as the entry point into BridgeDB's main run
   loop in `scripts/bridgedb`.
 * RENAME `bridgedb.Main.startup` to `bridgedb.Main.run` so that it is now the
   default entry point.
 * UPDATE docstring for `bridgedb.Main.run`.
 * UPDATE docstring for `bridgedb.configure.loadConfig`, which referenced the
   now removed `bridgedb.Main.startup`.
Rewrite b.p.d.parseNetworkStatusFile() to use RouterStatusEntryV3.

See Damian's comments on how we were using a RouterStatusEntryV2 in
https://trac.torproject.org/projects/tor/ticket/9380#comment:16.
Fix unittests for return values of parseNetworkstatusFile().

`bridgedb.parse.descriptors.parseNetworkstatusFile()` now returns list
of `stem.descriptors.router_status_entry.RouterStatusEntryV3s`.