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.

Don't declare and then assign; use the := operator.