Added support the HTTP Basic authentication scheme for client authN

I added a support of Basic authN scheme for the client authN at Token Endpoint.

[2.3.1.  Client Password - The OAuth 2.0 Authorization Framework](http://tools.ietf.org/html/rfc6749#section-2.3.1)

> The authorization server MUST support the HTTP Basic authentication scheme for authenticating clients that were issued a client password.