Share basepoints

This is dependent on #629, and fixes a bug I didn't enter.

Background:
* We now have a button that will allow people to download their data from Khan Academy, directly into their KA Lite installation.

Issue:
* However, our Video and Exercise points calculations are not localized, and so some hack-ish algorithm was previously used to compute points.

Changes:
* Each exercise has it's own "base" # of points per correct exercise.  Move this computation out of client code, and into khanload (so it can be updated based on KA topic tree changes, and can be shared across our application)
* Make functions for computing video # of points and exercise # of points
* Call those functions from the API for downloading the KA data.
* Call the video function from main/api_views.py.  
* Note: The exercise functionality is more embedded, so I left it alone, and duplicated the logic for now.

Testing:
* I manually tested exercise point calculations when doing exercises (still look good) and updates based on downloading KA data (made sense).
* I was too lazy to check whether videos work..........