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
Fix line wrapping in Gengo warning emails

The Gengo warning emails were running the text through textwrap.wrap
which ditches all the newlines before wrapping. This fixes that by
creating a wrap_with_paragraphs utility function that keeps the
intended paragraphs intact.
Fix subject line for gengo warning emails
Merge new code from master