Adding readygraph extension

To combine ReadyGraph feature to another project, please follow the instruction:
1. Copy 'extension' folder to the root of destination
2. Copy 'readygraph-extension.php' to the root of destination
3. For the main entry file, paste the following to the end of file:
    include "readygraph-extension.php"
4. Configure "readygraph-extension.php" file
    $readygraph_email_subscribe = <<<EOF
         function subscribe(email, first_name, last_name) {
              // Manage the user for mailing list (for example)
         }
    EOF;
    and don't forget to change the plugin name:  $main_plugin_title
5. Done