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.

Standard method comments start with the name of the method. Please change this to "RemoveNode removes all quads...".

(Yes there are counter examples throughout - they will eventually be cleaned up).