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.

This feels wrong. (Now that I think of it, the pre-allocation in your code below shows this). Return the []graph.Delta, since if you append past cap, those appends will not be visible to the outside world.