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
<!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>