Add notes about running tests with additional warnings

This will help us pick up 2to3 issues, tab issues and deprecation
warnings. Knowing about and fixing these makes upgrading to new
versions of things easier.
Fix mimetype -> content_type and int division issue

* Django deprecated mimetype in favor of content_type, so I fixed that
* we had one case of int division which I changed to be explicitly
  int division using //
Merge pull request #372 from willkg/code-cleanup

Code cleanup