Add of a requirements.txt file dedicated to dev

I added a `requirements.txt` file that is dedicated to the dependencies
required for the development part. You'll find in it among other things
libraries for testing, code coverage, syntax checking, documentation
generation, ...

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>

django/requirements_dev.txt

+pip>=9.0.2
+Sphinx==1.7.2
+sphinxcontrib-napoleon==0.6.1
+bumpversion==0.5.3
+flake8==3.5.0
+coverage==4.5.1
+tox==2.9.1
+pytest==3.4.2