Add support for Oauth2 "Authorization Code" authorization grant

[chriskapp]

At the moment the Oauth endpoint supports only the "Resource Owner Password Credentials" authorization grant. We need also support for "Authorization Code" so that the API can be easily used by web apps. Therefor we must provide a way for an consumer to login and grant an app authorization / manage authorized apps/scopes etc. This should be probably a login seperated from the backend. Also it must be easy to customize the design of the "consumer" backend and to connect the login to a 3rd party system because in most cases the user uses credentials from another system.

[chriskapp]

Implemented in the 0.2 release