Serialize tag attributes in a stable order

`attributes.iteritems()` doesn't have a guaranteed order and depends on
the underlying implementation.
This caused some tests to fail under specific circumstances because they
were expecting a specific order which wasn't the one returned in this case.

This fix makes attributes serialization stable, and fix some tests which
were expecting a different order than the new, stable, one.

Fix #50
Isolate the doc building process from the tests through Tox