first pass at WCT implementation
fix fancy getters that didn't work

Good explanation here http://stackoverflow.com/a/2982/744071.
Could use types.MethodType but that's too complicated.
nodes_in_preorder method for testing
add eidma figure 3.2 as a test

Turns out we can't precompute a KT table, because the indices in this
table are not bounded by MAX_DEPTH, but by the length of the document,
which could be huge. So we need to compute the KT updates as we update
the tree.