Support non-string node attributes

[peterl-stripe]

PR 16 adds support to add nodes and set string attributes, but doesn't add support for non-string attributes. In particular
client.set_node_attribute(daniel, "login_count", 42)
isn't implemented yet in either ruby or python client tests.

One approach is add code on the client side to convert a non-string to a JSON string (similarly to how we handle get_node_attribute) and add code in the scala database client to deserialize the JSON and handle string, int, and bool properly.