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.
I don't love that this has to be handled manually - maybe it can happen in the #key method of the PublicKey class?