Added support for 500px.com. Also removed the '@Override' annotation from JsonTokenExtractor.java

src/main/java/org/scribe/builder/api/Px500Api.java

@@ -0,0 +1,26 @@
+package org.scribe.builder.api;
+
+import org.scribe.model.Token;
+
+public class Px500Api extends DefaultApi10a
+{
+  private static final String AUTHORIZATION_URL = "https://api.500px.com/v1/oauth/authorize?oauth_token=%s";
+  
+  @Override
+  public String getAccessTokenEndpoint()
+  {
+    return "https://api.500px.com/v1/oauth/access_token";
+  }
+
+  @Override
+  public String getRequestTokenEndpoint()
+  {
+    return "https://api.500px.com/v1/oauth/request_token";
+  }
+  
+  @Override
+  public String getAuthorizationUrl(Token requestToken)
+  {
+    return String.format(AUTHORIZATION_URL, requestToken.getToken());
+  }
+}