_Session.set_variable for ndarray and spmatrix

This function will correctly serialize numpy arrays as json due to
customizations to the JSON encoder. Additionally, this function will
handle scipy sparse matrices as well.

The matlab magic now calls this function.

This commit *DOES NOT* add any new dependencies to numpy and scipy, and
the code should function as normal without them.
@aebrahim : given these examples, I agree that a global counter variable on the python side wouldn't help, because presumably you might have two instances of the pymatbridge connecting to the same instance of the matlab server. 

@isbadawi's UUID suggestion is probably our best bet. It also tells whoever reads the code that this is here specifically to ensure uniqueness.