Beta version of a Gradebook.

Gradebook view renders student login names vs. assignment scores. Columns are all sortable asc/desc.

Some more things to do:
* Link the column heading name to the student progress view containing statistics for all students for the set corresponding to the column heading name.
* Link the row label name (login name) to the student progress view containing statistics for all sets for the student with login name corresponding to the row label name.
* Column headings for sets should be sorted, at least alphabetically.  Now it just returns some random order.
* Add a column for totals (in percent?)


Questions:
* Should we make certain CRUD functions available on this page, like delete assignment or delete student?
 - for example highlighting a column and pressing a delete button removes the set for all users, or have an icon in the column heading that when pressed, deletes the set for all users.  More precisely, for columns, we can add an icon that gives a dropdown menu of column specific functions like sorting and including some CRUD functions.  For example, consider an instructor viewing the GradeBook that decides they want to delete a set.  They'll click an icon next to that set and select delete from corresponding dropdown menu.  This would involve less clicks than navigating to the problem set manager, and clicking the delete button.
 - Similarly for deleting students (GradeBookRows). Perhaps something like this:
![rowcrud](https://cloud.githubusercontent.com/assets/3240137/4855970/12fa6e42-60b0-11e4-891c-91c96596942c.png)
 - Can we extend the CollectionTableView to include a footer for say, set averages? But this is column specific, so maybe include it in the dropdown menu suggested above.