Added binding for HoudiniScene::node()

This gives access to the underlying Houdini node. Note that we aren't currently return a PyObject* directly, because Houdini only provides a mechanism for that using swig, which we don't build. I've requested that SideFx provide a swig-free approach, and if they do, we should fix this up.
Added HoudiniScene::embedded() method and bindings.

This returns a bool indicating if the scene is pointing to hierarchy embedded inside a SOP.
Merge pull request #141 from andrewkaufman/houdiniSceneNodeBinding

HoudiniScene node() and embedded()