Leaflet easybutton binding

<!DOCTYPE html>
<html>
  <head>
    <link rel="stylesheet" href="//cdn.leafletjs.com/leaflet-0.7.5/leaflet.css" />
    <link rel="stylesheet" href="https://raw.githubusercontent.com/CliffCloud/Leaflet.EasyButton/master/src/easy-button.css" />
    <link rel="stylesheet" href="http://netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css" rel="stylesheet" />
    <link rel="stylesheet" href="style.css" />
  </head>
  <body>
    <div id="leaflet"></div>
    <script type="text/javascript" src="//cdn.leafletjs.com/leaflet-0.7.5/leaflet.js"></script>
    <script type="text/javascript" src="https://rawgit.com/CliffCloud/Leaflet.EasyButton/master/src/easy-button.js"></script>
    <script type="text/javascript" src="script.js"></script>
  </body>
</html>

var map = L.map('leaflet', {
  'center': [0, 0],
  'zoom': 0,
  'layers': [
    L.tileLayer('http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
      'attribution': 'Map data &copy; OpenStreetMap contributors'
    })
  ]
});

var helloPopup = L.popup().setContent('Hello World!');

L.easyButton('fa-globe', function(btn, map){
  helloPopup.setLatLng(map.getCenter()).openOn(map);
}).addTo(map);

body {
  margin: 0;
}
html, body, #leaflet {
  height: 100%;
}