Added exceptions to both token methods on 2.0 service

Rebased 1.0.9

src/main/java/org/scribe/builder/api/GoogleApi.java

@@ -19,13 +19,13 @@ public class GoogleApi extends DefaultApi10a
   }
 
   @Override
-  protected Verb getAccessTokenVerb()
+  public Verb getAccessTokenVerb()
   {
     return Verb.GET;
   }
 
   @Override
-  protected Verb getRequestTokenVerb()
+  public Verb getRequestTokenVerb()
   {
     return Verb.GET;
   }

src/main/java/org/scribe/builder/api/LinkedInApi.java

@@ -1,17 +1,19 @@
 package org.scribe.builder.api;
 
+import org.scribe.model.*;
+
 public class LinkedInApi extends DefaultApi10a
 {
   private static final String AUTHORIZE_URL = "https://api.linkedin.com/uas/oauth/authorize?oauth_token=%s";
 
   @Override
-  protected String getAccessTokenEndpoint()
+  public String getAccessTokenEndpoint()
   {
     return "https://api.linkedin.com/uas/oauth/accessToken";
   }
 
   @Override
-  protected String getRequestTokenEndpoint()
+  public String getRequestTokenEndpoint()
   {
     return "https://api.linkedin.com/uas/oauth/requestToken";
   }

src/main/java/org/scribe/oauth/OAuth2ServiceImpl.java

@@ -23,13 +23,13 @@ public class OAuth2ServiceImpl implements OAuthService
   @Override
   public Token getAccessToken(Token requestToken, Verifier verifier)
   {
-    return null;
+    throw new UnsupportedOperationException("Unsupported operation, please use 'getAuthorizationUrl' and redirect your users there");
   }
 
   @Override
   public Token getRequestToken()
   {
-    throw new UnsupportedOperationException("OAuth 2 does not use request tokens. Use 'getAccessToken' directly");
+    throw new UnsupportedOperationException("Unsupported operation, please use 'getAuthorizationUrl' and redirect your users there");
   }
 
   @Override