Checkout oauth

Backend for oauth

Sets up a redirect that refers users to /checkout with a session token

Sets up a new implicit grant in oauth. If the session token is set, it will issue a implicit grant and jwt for the logged in user

Added an api that writes sendwyre user accounts to a cassandra table that uses the user's guid as a primary key.

We'll need to set the "checkout_url" to the base hostname of the checkout site.

#261 (closed)