Add client credentials authentication filter

Fixes gh-5

core/src/main/java/org/springframework/security/oauth2/server/authorization/authentication/OAuth2ClientAuthenticationToken.java

@@ -44,11 +44,11 @@ public class OAuth2ClientAuthenticationToken extends AbstractAuthenticationToken
 
 	@Override
 	public Object getCredentials() {
-		return null;
+		return this.clientSecret;
 	}
 
 	@Override
 	public Object getPrincipal() {
-		return null;
+		return this.clientId;
 	}
 }