Missing cssselect and mock requirements.

The cssselect module was missing from the requirements.txt. The mock module is needed to run one test in the tests suite, that module was already added to the travis' requirements but is missing from any requirements file in the root directory.

As is common in another projects, the `requirements-tests.txt` was created to specifically hold the modules required for running the tests. This does not include optional modules.

Also, `.travis/requirements-lucid.txt` was not modified as python 2.6 support is going to be dropped and it's most likely cssselect does not work with lxml 2.2.x.