_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.
Sorry, I didn't make the pull request -- I'm just drive-by commenting. I'm pretty sure he is using it for uniqueness.