Add support for create actions in the node client

- Adds validation to require `_id` for create actions (including for the http protocol)

**Help**: I don't know how to properly test for the error condition. The `"create action fails without _id"` test fails because I'm not testing it right, but I don't know how to properly catch the error.

Note: #2 (retry on bulk failure) may need to special case failure to create (ignore it).

Closes #28