The Resource Owner Password Credentials Authorization Strategy
@see tools.ietf.org/html/draft-ietf-oauth-v2-15#section-4.3
Not used for this strategy
@raise [NotImplementedError]
Retrieve an access token given the specified End User username and password.
@param [String] username the End User username @param [String] password the End User password @param [Hash] params additional params
[Validate]