You can try this syntax
<!DOCTYPE html>
<html>
<head>
<title>Leaflet Quick Start Guide Example</title>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link href='https://api.tiles.mapbox.com/mapbox.js/v1.6.2/mapbox.css' rel='stylesheet' />
</head>
<body>
<div id="map" style="width: 1000px; height: 1000px"></div>
<script src='https://api.tiles.mapbox.com/mapbox.js/v1.6.2/mapbox.js'></script>
<script>
var map = L.mapbox.map('map');
L.tileLayer('http://eccoilmoro.github.io/Mappa-Redditi-IRPEF-2012-/tiles-mappa-redditi-2012/{z}/{x}/{y}.png', {
"attribution": "franco"
}).addTo(map);
map.setView(new L.LatLng(39.5, 9), 6);
</script>
</body>
</html>