Changed the getAccessToken() method implementation to conform to the OAuth 2.0 server-side flow

src/main/java/org/scribe/oauth/OAuth20ServiceImpl.java

@@ -1,7 +1,12 @@
 package org.scribe.oauth;
 
-import org.scribe.builder.api.*;
-import org.scribe.model.*;
+import org.scribe.builder.api.DefaultApi20;
+import org.scribe.model.OAuthConfig;
+import org.scribe.model.OAuthConstants;
+import org.scribe.model.OAuthRequest;
+import org.scribe.model.Response;
+import org.scribe.model.Token;
+import org.scribe.model.Verifier;
 
 public class OAuth20ServiceImpl implements OAuthService
 {
@@ -35,7 +40,13 @@ public class OAuth20ServiceImpl implements OAuthService
    */
   public Token getAccessToken(Token requestToken, Verifier verifier)
   {
-    throw new UnsupportedOperationException("Unsupported operation, please use 'getAuthorizationUrl' and redirect your users there");
+    OAuthRequest request = new OAuthRequest(api.getAccessTokenVerb(), api.getAccessTokenEndpoint());
+    request.addQuerystringParameter(OAuthConstants.CLIENT_ID, config.getApiKey());
+    request.addQuerystringParameter(OAuthConstants.CLIENT_SECRET, config.getApiSecret());
+    request.addQuerystringParameter(OAuthConstants.CODE, verifier.getValue());
+    request.addQuerystringParameter(OAuthConstants.REDIRECT_URI, config.getCallback());
+    Response response = request.send();
+    return api.getAccessTokenExtractor().extract(response.getBody());
   }
 
   /**