title: Wordpress Plugin Upgrade Hook Function
body: <p>I'm developing new version of my wordpress plugin (<a href="http://wordpress.org/extend/plugins/facebook-send-like-button/" rel="noreferrer">http://wordpress.org/extend/plugins/facebook-send-like-button/</a>) . </p>
  
  <p>New options (<code>add_option()</code>) coming with new version. But i can't register this new options automatically.</p>
  
  <p>For example there is <strong>fgb_single</strong> option in new version.
  Where should i put <code>add_option('fgb_single', 'on');</code>  code in my plugin's file?</p>