Add markdown as a dev requirement.
This allows everyone to have the same setup when generating the tests. It is
useful for #288 for instance.
Add mock to dev_requirements.
Add pytz in requirements
Add the dev_requirements.txt file. Fix #195
removing reqs.txt file: replaced by tox
Update the "how to contribute" document.
Also remove an unwanted dep to Markdown and add a dev_requirement.txt file to use with pip.