Student progress reports for playlists

Not quite ready to merge, but putting it up here to get some feedback, mostly on the UI but if @aronasorman or @rtibbles have a second to look at the mess I've created with tastypie, that would be helpful too :+1: @jamalex is always welcome anywhere in my code as well, of course :smile: 

UI screenshots:
![selection_055](https://cloud.githubusercontent.com/assets/1319950/3975324/795b8916-2805-11e4-8372-06093877a62c.png)
![tooltip_058](https://cloud.githubusercontent.com/assets/1319950/3975325/7e964c7c-2805-11e4-8cec-f9c2d1390cc8.png)



Remaining issues:

1. API authorization -- only admins with permission and the student themselves should be able to view their report
2. Gnarly messy code in the coachreport model and api resources. See if some of it can be abstracted to be more general
3. Need to write tests 

Note to self, things to test:

1. Generate more test data and look for weirdness. 
2. Test that you can view these reports from the coach view (e.g. not from account management page) 
3. How does it look on tablet sized screens? How about mobile?

UX issues (fix them):

1. No way to click through to the playlist, individual exercise, or video yet
2. Are the colors meaningful enough? Should I add in descriptions of what the progress bar and the scores mean? (probably, especially b/c we have the Handlebars helper to grab the strings) 
3. Flip the caret upside down when expanding out the details 

good thinking, updated to:

```python 
elif ex_pct_struggling > 0:
    # note: we want to help students prioritize areas they need to focus on
    # therefore if they are struggling in this exercise group, we highlight it for them 
    ex_status = "struggling" 
```