Added specific constants for OAuth 2.0

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

@@ -41,4 +41,9 @@ public class OAuthConstants
 
   //OAuth 2.0
   public static final String ACCESS_TOKEN = "access_token";
+  public static final String CLIENT_ID = "client_id";
+  public static final String CLIENT_SECRET = "client_secret";
+  public static final String REDIRECT_URI = "redirect_uri";
+  public static final String CODE = "code";
+  
 }