Added OAuth 2.0 classes

src/main/java/org/scribe/model/OAuthConstants.java

@@ -38,4 +38,7 @@ public class OAuthConstants
   public static final String HEADER = "Authorization";
   public static final Token EMPTY_TOKEN = new Token("", "");
   public static final String SCOPE = "scope";
+
+  //OAuth 2.0
+  public static final String ACCESS_TOKEN = "access_token";
 }