OATH2 Username & Pwd access support

I have an application where I want to use OATH2 Username & Pwd access by having a client side login form and posting a JSON object with credentials to the server. The description of the workflow I have at hand can be seen on the [Apigility website](https://apigility.org/documentation/auth/authentication-oauth2#username-and-password-access).

I really like your implementation of the oauth-ng module and thought I'd contribute support for this rather than writing my own module. This branch is not ready for a pull into the master yet, but I wanted to get your feedback on whether you would consider adding this work to the module. At the moment you can use the `<oauth-pwd>` directive (with parameters deduced from the source) to display a login form and once a success result is obtained the `AccessToken` service is set as before and events triggered. 

If you'd be interested in a merge I'll complete the implementation, clean it up and let you know when the branch is ready.