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 agree order is unlikely to be important (but tests are required). Since that is the csae, please just use a counter over the graph.Directions.