Add support for default graph

[chrhansen]

Retrieving Default Graph
It should be possible to retrieve a default graph See Python doc. This might be the first thing to build as the code for creating ops and running a session would use the "get-default-graph" method.

Creating Ops
Unless explicitly specified nodes should be added to this default graph:

Tensorflow.constant([1.0, 2.0, 3.0], shape: [3], name : 'a')
# or short-hand:
TF.constant([1.0, 2.0, 3.0], shape: [3], name : 'a')

Session
Also, when Session.run is called it should grab the default graph if not explictly specified
https://www.tensorflow.org/versions/r0.10/api_docs/python/client.html#Session.run

Python Graph documentation: https://www.tensorflow.org/versions/r0.10/api_docs/python/framework.html#Graph

[chrhansen]

@geoffreylitt didn't you mention at some point you wanted to look into adding support for default graph?

[geoffreylitt]

Yep! Earlier discussion was here: #52

Last couple weeks have been busy and I haven't had time to make progress on an implementation, but I was planning to work on this in the next week or so.