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>
[`Node.serialize`](https://github.com/cbmi/origins/blob/master/origins/backends/base.py#L158-L167) takes a few arguments to augment the properties. In this context, this does not make sense, so they should all be set to false. However, the `Node.label` property can be added to the component dict itself (not in `properties`). This serves as a consistent attribute for component labels.