Make the hub_base_url configurable.
Make the hub_base_url configurable.

alope107/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,modulexcite/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,alope107/nbgrader,jupyter/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,modulexcite/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,dementrock/nbgrader,jdfreder/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,MatKallada/nbgrader,jdfreder/nbgrader,MatKallada/nbgrader