Make flake8-run.sh only require FLAKE=true if TRAVIS=true.
Reorder HTMLSerializer options to match definitions
Add alphabetical_attributes option to the serializer
Clean up comments/whitespace around filters for clarity.
Use alphabetical_attributes option in tests.
Fix warnings xgettext gives in html5lib.

This is relatively trivially resolved by changing the bare `%s`,
`%r`, etc. into named substitutions.
Fix #6: dom2sax crash by replacing dom2sax with a generic to_sax

This moves the functionality to a new treeadapters module (where
later the adapters from test_treewalker.py will get moved) and
removes the previous dom2sax function.
Add test for sax treeadapter.
Fix #63: treewalker fails to handle bytes data from outwith html5lib

We do, ourselves, ensure everything we put in the tree is unicode
under Python 2; users, however, may not be so careful and assign
an attribute using a bare string, for example. We should handle
this gracefully under Python 2 by coercing gently into unicode,
failing in the normal way if we are unable to decode the bytes.

I (Geoffrey, the committer) am not entirely convinced this is the
right way to test this; on the other hand, a thorough testsuite for
the issues previously in the treewalker would be large, especially
considering the large number of treewalkers we support.
Update changelog for 1.0b2 changes.
Add fantasai, Marc, and Mike as contributors, and note the order.