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>
The `resource` property should be [`self.uri`](https://github.com/cbmi/origins/blob/master/origins/backends/base.py#L135-L138) (fully qualified) and the ids should be the [`self.path`](https://github.com/cbmi/origins/blob/master/origins/backends/base.py#L129-L133) which is relative to the top-level component (resource). The ID of a relationship should be created based off the start, end and type, e.g. `<start_path>:<type>:<end_path>`.