Merge pull request #93 from dulaccc/fix-changed-by-not-set

Fix tests after changes introduced by #86 Thank you
Fix image diff tests
Merge pull request #94 from dulaccc/enhance-admin-layout

Improve the image diff layout with a little css. Thank you @dulaccc
Add a Makefile for convenience on running pep8 tests
Add the venv usual directory to avoid adding the virtualenv to the repo
Replace the EmailThread by backend classes that are switchable

currently only support the synchronous email backend