Assignment toolbar

Add javascript and css for the nbgrader toolbar for creating assignments.
IIRC if you require `require` ([example](https://github.com/ipython-contrib/IPython-notebook-extensions/blob/master/styling/css_selector/main.js#L2)) you can ask require for its full path [like here](https://github.com/ipython-contrib/IPython-notebook-extensions/blob/master/styling/css_selector/main.js#L39) and allow you to link to relative path css, which is easier to keep extension in only one folder.