Testing documentation build

This pull request introduces tests for the documentation:

* added a `test` target to project's Makefile
* fixed documentation so that tests actually pass
* added a "use doctests" recommendation... and applied it to docs.writethedocs
* introduced a .travis.yml for use with https://travis-ci.org/ => **needs workaround**: enable TravisCI hook on Github repository, check that the provided .travis.yml works fine (I didn't test it).

If the pull request is merged (and TravisCI enabled), we will get "documentation build" status, including for pull requests. Contributors will be committed to build success (not only involved into).