Allow cancelling of XHR downloads
Merge pull request #59 from max-li/cancel_download

Allow cancelling of XHR downloads
Add Max Li as contributor to README
Merge pull request #60 from max-li/readme

Add Max Li as a contributor to the README
Add in fetch for Perseus exercise when exercise loads
Add support for fetching random assessment item in a perseus exercise

This just displays the question content as is without converting it. It
looks like it's markdown. I'll work on rendering that properly in a
followup diff.

Test Plan:
- Load an exercise, go back and forward in navigation and make sure it
  changes the item loaded each time.
Add perseus submodule
Update RequireJS
Update README
Start to render perseus exerciess

Crude and hardly works, but it's a start

Test Plan:
- None
Add katex lib dep
Add perseus build output

This adds the folders/output of what make subperseus requires.

Test Plan:
- None