[GA-153-0] Implement NodeDict update method

[hkernbach]

Replaces #12

[aMahanna]

Looks good! just one small thing;

Can we add this update method in the CustomNodeView object? This will allow us to users the following:

G.nodes.update(...)

Instead of having to reference the _node object:

G._node.update(...)

[aMahanna]

minor things, looks good overall

[hkernbach]

Looks good! just one small thing;

Can we add this update method in the CustomNodeView object? This will allow us to users the following:

G.nodes.update(...)

Instead of having to reference the _node object:

G._node.update(...)

Added in dce4e85