added L.UtfGrid class factory in Leaflet 0.4 style

In mid-2012, the Leaflet project [reworked how Leaflet objects can instantiated in the library](http://leafletjs.com/2012/07/30/leaflet-0-4-released.html), making the use of `new` optional.

I have added this style to Leaflet.utfgrid. Now, this:

    var grid = new L.UtfGrid('http://{s}.tiles.mapbox.com/v3/...')

Can work like this, making chaining more feasible/pretty:

    var grid = L.utfGrid('http://{s}.tiles.mapbox.com/v3/...')