Support for running tests in Travis CI

Test against more python versions and lxml as well. The build for
current lxml seems to be broken.

Signed-off-by: Michal Čihař <michal@cihar.com>
Add support for XML testing

Test reformatted xml, so that we don't fail in case attributes are
generated in different order.

Signed-off-by: Michal Čihař <michal@cihar.com>