Run tests with several versions of docutils

[benoitbryon]

#18 introduces support for both docutils 0.10 and 0.9.1.

We need to run tests with all the versions...

... or explicitely require docutils >= 0.10. But since #18 supports both, let's do it :)

Try https://pypi.python.org/pypi/tox/ and see whether it makes it easy.
Or, maybe, propose another tool.

=> As a developer, in order to make sure code works with several versions of docutils, I want make test to run tests against several environments.