Replaced existing oauth2:issue_code_grant functions

After finishing my implementation of the Authorization Code Grant flow, I came to the conclusion that none of the existing oauth2:issue_code_grant functions is really helpful. None of them verifies the resource owner's username or password, nor the scope. issue_code_grant/5 doesn't comply with the specification because it expects a client secret that is not in the request. See http://tools.ietf.org/html/rfc6749#section-4.1.1.