Assignment toolbar

Add javascript and css for the nbgrader toolbar for creating assignments.
Because cells already have a `cell_type` attribute in the document format, I find that using that name here is a bit confusing. Originally I was expecting to have two separate boolean attributes `solution` and `grade`. Trying to think if you would ever want a cell that was both solution and grade. Actually I think there is one. If you asked the students to write the tests themselves. Then you would want both to stub the cell in the student version, but still autograde it later. Based on this, I think we should use `metadata.nbgrader.solution=true/false` and `metadata.nbgrader.grade=true|false`. Then the UI for each would be a checkbox. Does this make sense?