Fix optionaltags filter to not error when filtering nothing.
Update lint filter for Py3 and namespaced attributes.
Fix #72: rewrite the sanitizer to be a treewalker filter only.

As we no longer need the sanitizer to be shared between a filter and
a tokenizer, move the entire sanitizer to the filter module.

Also, replace the existing, tiny sanitizer testsuite with the one
in html5lib-tests.