Merge branches 'timezone', 'onlyletters' and 'tox' into develop
Move test code to a separate root dir
Adjust test runner parameters
Fix imports and references to the test app
Refactor mock files
Remove unused import
Enable runtests be executed as a library

Setup.py needs a callable to execute tests on 
`python setup.py test`. So the command line parsing
needed to be decoupled from the runtests function.
Enable setup.py test command
Remove PIL and tox as dependencies

They are 'test dependencies', so we let setup.py
handle it.
Feed setup.py with requirements.txt content
Update instructions for hackers.
Fix setup.py install_requires parameter