Add __all__ to utils.py to explicitly export default_etree import

This silences flake.
Fix flake8-run.sh to return non-zero on failure.
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.