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.
Somehow merging left a redundant import; remove this.
Clean up getTreeBuilder, remove dom.TreeBuilder from 0.10 legacy.
Move testdata submodule to using HTTPS.

This provides read/write access to those of us with write access
without forcing everyone to authenticate to read it.