Added IE11 native fullscreen support

There seems to be a problem with the popup content (for a marker) in IE11 when going fullscreen and leaflet inside an iframe.
The popup is not positioned directly above the marker, and the popup width is not set correctly.

For a test case, the basic example from http://leafletjs.com/ can be used together with an IFrame
```html
<!DOCTYPE html>
<html>
	<head>

	</head>
	<body>
		<div style="height: 630px; width: 710px;">
			<iframe src="index.html" allowfullscreen="" webkitallowfullscreen="" mozallowfullscreen="" onmousewheel="false" frameborder="0" allowtransparency="true" style="height: 100%; width: 100%;"></iframe>
		</div>
	</body>
</html>
```

```html
<!DOCTYPE html>
<html>
	<head>
		<link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.css" />
		<link rel="stylesheet" href="leaflet.fullscreen/leaflet.fullscreen.css" />
		<script src="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.js"></script>
		<script src="leaflet.fullscreen/Leaflet.fullscreen.min.js"></script>
		<style>
			html, body {
				height: 100%;
			}
			
			#map {
				width: 100%;
				height: 100%;
			}
		</style>
	</head>
	<body>
		<div id="map" style="">
			
		</div>
		<script>
			// create a map in the "map" div, set the view to a given place and zoom
			var map = L.map('map', {
				fullscreenControl: true
			}).setView([51.505, -0.09], 13);

			// add an OpenStreetMap tile layer
			L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
				attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
			}).addTo(map);

			// add a marker in the given location, attach some popup content to it and open the popup
			var marker = new L.Marker([51.5, -0.09]);
			marker.addTo(map)
				.bindPopup('A pretty CSS3 popup. <br> Easily customizable.');
		</script>
	</body>
</html>
```
A comment like this is better as a GitHub issue than a code comment.