Implement graph export method on Node

An export method is added on the Node class which returns a json object
that represents the graph of the node instance and its definitions,
recursively. The return value is as speficified in issue #38 with a
format version number, the id of the top-level node, and dicts of nodes
and relationships, each with an arbitrary but consistent id, a type, and
a dict of properties. To support this method, __eq__ and __ne__ methods
are added to the Node and Rel classes and a get_map_id utility method is
added (in utils) that iterates over {`id`: `obj`, ...} type dicts and
returns the id of the passed object or a new, unclaimed id and a boolean
representing whether or not the id is new as a two-dict.

Fixes #38
Fixes #54

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>
This is based off the [Serializer](https://github.com/bruth/graphlib/blob/master/graphlib/serializer.py#L12) class I wrote in graphlib that also supports incremental serialization (or in this case exporting) of objects. For example, I could connect to a backend and export a few tables with one exporter instance rather than merging them in myself. Most of the logic is in place, so we might as well use it.