Tidied up Node bindings.
Tidied up Node bindings.

- Removed scriptNode() wrapper function and replaced with CastToIntrusivePtr.
- Moved implementation details to unnamed namespace.
- Removed redundant includes.

GafferHQ/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,boberfly/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,chippey/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,goddardl/gaffer,chippey/gaffer,johnhaddon/gaffer,hradec/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,boberfly/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,hradec/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,cedriclaunay/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,goddardl/gaffer,andrewkaufman/gaffer,cedriclaunay/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,chippey/gaffer,hradec/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,goddardl/gaffer,boberfly/gaffer,chippey/gaffer,goddardl/gaffer,ImageEngine/gaffer