Fix error when desc digest parsed from networkstatus is None.

 * FIXES a bug where, if the nickname was not to spec (longer than 19
   characters), then the nickname and descriptor digest are both set to
   ``None`` and ``bridgedb.Bridges.parseStatusFile()`` attempts to
   format the null descriptor digest into hexadecimal.
Merge branch 'hotfix/9264-descdigest-none' into develop
Add draft of volunteer page text for GSoC 2014 applications.