Login with password grant using /oauth/token endpoint

[lbalmaceda]

Allow to log in with Resource Owner using /oauth/token endpoint.

Usage:

AuthenticationAPIClient client = new AuthenticationAPIClient(getAccount());
client.login("username", "password")
    .setAudience("https://domain.auth0.com/my/api")
    .setScope("profile friends:email")
    .start(callback);  //Callback will receive `Credentials` on success.