Use BeautifulSoup instead of lxml (issue #284)

**Do not merge yet!**

This PR changed the library from lxml to bs4. A problem is that when calling the ``query_survey`` method is called, ``InconsistentTableError`` is raised because for some reason the returned content ``survey_response`` is some HTML that displays the form again and is not CSV data as expected. Maybe someone of you can see what's wrong there?