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.

Author: gsnedders

CHANGES.rst

@@ -28,6 +28,8 @@ Released on XXX
 
 * Cease supporting Python 3.2 (in both CPython and PyPy forms).
 
+* Sanitizer change XXX
+
 
 0.9999999/1.0b8
 ~~~~~~~~~~~~~~~