Insomnia-fueled stab at Unicode functionality.

BEFORE: I had not implemented any of the platform-hiding functions that
projects in the wild need to easily support Jython *and* do their tests for
funky Unicode values.

AFTER: I implemented 3 functions that I think would cover all of
html5lib-python's needs for this type of thing: scrub_invalid_unicode,
find_invalid_unicode, and find_invalid_unicode_iter.