Name subrings after their parent hashring's distributor.

 * MOVE logic for parsing filternames from the "unique string" in method
   FilteredBridgeSplitter.addRing() to new method
   FilteredBridgeSplitter.extractFilterNames().
 * ADD a FilteredBridgeSplitter.distributorName attribute which is set
   when Dist.Distributor.setDistributorName() is called.
 * REFACTOR FilteredBridgeSplitter.addRing() to use the new subring names.
Log when we dump assignments to a file.
Remove extraneous log statement from FilteredBridgeSplitter.prepopulate().
Don't log the hashring filters twice, just log the number of bridges inserted.
Update bridgedb.parse.networkstatus module docstring.
Fix a potential bug where an OR ID might have been unprocessed base64.

 * CHANGE bridgedb.parse.networkstatus.parseRLine() to check the OR
   nickname validity only after parsing (and readding the base64 padding
   to) the OR ID digest.
 * ADD inline comments about why this is done.
Add coding: utf-8 header to lib/bridgedb/Bucket.py.
Log when we're dumping bridge assignments to a file in bridgedb.Bucket.
Fix bug where open() was called even when COUNTY_BLOCK_FILE=None.
Call Distributor.setDistributorName() in subclass __init__s.
Log when a distributor's hashrings are being prepopulated.
Update docstring parameters for IPBasedDistributor.getBridgesForIP().
Change loglevel on message about adding PTs to a bridge in bridgedb.Main.
Cleanup the log statements for hashring prepopulation in bridgedb.Main.