Pin the version of html5lib

Reason is that the simpletree walker, used by bleach, was removed in
html5lib 1.0b1. See
https://github.com/jsocol/bleach/issues/94
Small updates to the test suite
Add new decorator to supress the post_save signal

This is needed e.g. when loading fixtures for user profiles so that
there is no conflict with the data from the fixture and the
automatically created one.
Some changes to make the main.py script work with postgreSQL

Also, update the travis configuration to test the application with mysl
and postgresql as well.