Add /oauth2/token endpoint

[jbertovic]

Add the endpoint /oauth2/token so the following could be accomplished

1. With Authorization Code grant (after web authorization) could retrieve authorization_token and refresh_token
2. With refresh_token can renew authorization token
3. With refresh_token can renew refresh_token and authorization_token

Will need redirect_url (with 1) and customer_key (with 1, 2, and 3) as per authentication endpoint docs