Add build status image to readme
Add build status image to readme

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