Delete node api

Opening pull request for this issue:
https://github.com/google/cayley/issues/123

Which also references this pull request:
https://github.com/google/cayley/pull/136

Not ready for merge, but would be good to have an easy way to get feedback on code. Before merging, I would like to complete some tests and write the http end point.

I don't think direction is important, but I do like the idea of the directions being explicitly listed. So you would suggest having all the action in one method in double for loop (RemoveNode) over something like what I did above?