Implement id.webmaker.org OAuth2 integration

[toolness]

Lots still to be done here:

• Generate state when redirecting the user to id.webmaker.org and check it in our callback.
• Actually log the user in/out (will probably involve implementing a very simple Django authentication backend).

Note that completing this PR will also require the following to be resolved in id.webmaker.org:

• Need username and email from OAuth2 API id.webmaker.org#118 (Need username/email from API)
• /login/oauth/access_token seems to be a GET instead of a POST id.webmaker.org#116 (/login/oauth/access_token needs to work as documented)
• Document and/or implement logout API id.webmaker.org#119 (Document/implement logout API)
• Implement "action" argument for authorize endpoint id.webmaker.org#131 (Document/implement intent argument for authorize endpoint)

[toolness]

Ok, the intent/action part isn't done yet, and this code doesn't have a lot of tests yet, but I'm going to merge it and file issues for the stuff to fix.

[toolness]

Filed #13.