Add a test suite
Use tox to launch the test suite with different Python version
Update docstring of the 'compressor' template function

Initialyl, the `compressor` function was used with assets, but has been
modified to use bundles. The docstring now reflects the modification.
Fix the 'compressor' template function

The `compressor` template function was still using the
`get_linked_contents()` from the `Bundle` class. This function have been
removed in the favor of the use of the `concatenate` argument of the
`get_linked_content()` function;
Add the 'lesscess' processor