Current user

Added Rack middleware that verifies an access token from auth.prx.org, then takes the information encoded in that token and attaches it to the request. Also added a current_user method to BaseController that uses the information added to the request to return the user object of the user who belongs to that access token.
by hardcoding this here, we're guaranteeing that this will start failing in a year, so we should add a #TODO about generating an access token in some way for the test, or else also hardcoding in the certificate.