coveralls + testing and contributing doc

[DylannCordel]

PR to enhance the code quality of pybbm. Currently, it's difficult to find dead code or untested lines. With coveralls, you can have statistics and identify untested lines for python files. This PR:

• adds coveralls to tox and travis config files and coveralls badge in the README. (hovel organisation must sign-in for a free account on coveralls). From webu:features/coveralls branch, this badge looks like this:
• adds some pragma: no cache comments for lines which are not relevent to the code quality (deprecated code for exemple)
• enhances the documentation about testing and add a specific contributing page.

[DylannCordel]

Thanks for your feedback @skolsuper
I push another commit to fix typos and run coveralls only for django1.8 + postgres.

[lampslave]

hovel organisation must sign-in for a free account on coveralls

@zeus, what do you think about it? https://coveralls.io/