Fix #37: Preserve order of attributes on serialization.

This doesn't do anything about the fact that none of our
treebuilders preserve attribute order: it merely avoids the
serializer reordering them from the order it receives them in.

This also provides a filter to obtain the previous behaviour of
lexicographical ordering, used by the serializer tests to ensure
determinism of the output.
fixup! Remove dom2sax entirely.
Somehow merging left a redundant import; remove this.
fixup! Remove dom2sax entirely.
Move default_etree to utils.py and use it for the treewalker too.
fix the test again to work again
Merge remote-tracking branch 'html5lib/master' into dom2sax