Remove unused t.p.log.defaultObserver import, replace with variable.

 * REMOVE unused import twisted.python.log.defaultObserver from bridgedb.log
   module.
 * ADD bridgedb.log.defaultObserver from already imported t.p.log, and a
   comment explaining why the defaultObserver is necessary for logging events
   which occur before BridgeDB's actual logger is set up.
Remove unused import t.p.log.startLoggingWithObserver from log.py.
Change Sphinx docstring directive from an attr comment to a code comment.

Using '#: ' to start each line of a comment before an attribute with Sphinx
will autodoc the attribute.
See http://sphinx-doc.org/ext/autodoc.html#directive-autoattribute.