Add reverse proxy

To circumvent the same origin policy, I have created a simple reverse
proxy using node-http-proxy
(https://github.com/nodejitsu/node-http-proxy).

To initialize the entire system, be sure to call `npm install` followed
by `npm start`.

See #9.
Fix subtle bug with the factory for Submission

1) Ensure that the submission factory *must* be returned.
2) Fix typo for function name respondToChallenge().

See #9.
Make GET request to Coursera

This makes use of our reverse proxy to circumvent the same origin
policy for XMLHTTPRequest(). Code is in a bit of a mess since I don't
know of a good way to properly organize the callbacks.

See #9.