Feature/progress bar

I think it would be useful for students to have a visual display of their progress on their current Problem Set. At present, the side pane gives a list of problems - the hyperlinks are useful, but it seems like they could be made more useful.

This pull request adds a Progress Bar to the `fisheye div box` in the side pane. It has `green` for the number of `correct` answers, `red` (ish) for `incorrect`, and `nothing` for `unattempted`.  Furthermore, `checkmarks` are added to those questions that have been completed correctly, and `crosses` to those that have been completed incorrectly. The current problem is also highlighted, as shown below.

![screenshot from 2014-05-21 20 20 57](https://cloud.githubusercontent.com/assets/2224480/3049497/4b1ca35a-e160-11e3-88b8-0456f954acd2.png)

Each portion of the `progress bar` has a `ToolTip` detailing the information.

![screenshot from 2014-05-21 20 21 47](https://cloud.githubusercontent.com/assets/2224480/3049498/5234b592-e160-11e3-9f4b-710292a86445.png)

A global switch is available in the `Course Configuration` screen, which disables the `Progress Bar`. 

All feedback is greatly appreciated :)