[PR] Clean ups + fix a silly mistake in last PR

[Søren Løvborg]

FYI, and related to the earlier discussion, I'm manually "cross-posting" 
this pull request from Bitbucket to the mailing list.

https://bitbucket.org/conservancy/kallithea/pull-request/155/

Description:

     Clean ups + fix a silly mistake in last PR

     The first commit of this PR fixes a silly mistake I introduced while
     rebasing some of kiilerix's suggestions to my previous PR. The
     remaining commits are clean ups, many based on the discussions we
     had during the review of PR146 and PR150.

     I should note that I haven't tested commit 8605967 ("remove
     redundant hashlib imports"), since the affected code is Windows
     only (...?!), and such a setup is not readily available to me at
     the moment. I am, however, fairly confident that it's correct. :-)

     All other changes are covered by unit tests.

Best regards,
Søren Løvborg