Add username to getUserFromClient to make client_credentials work.

Client_credentials grant needs to also pass a username, otherwise is not possible to find just one user in the oauth model using just clientId and clientSecret, since the access_token relation to the user would return all users with that clientId. by doing this:

```
this.model.getUserFromClient(clientId, clientSecret, uname, function (err, user) {}
```

We allow the users of the module to define how they want to look for the user in oauth model, and allow them to actually identify just one user.