Factory function added for consistency with Leaflet conventions

Leaflet defines factory functions for classes for convenience (named after class constructors except for lowercase initial). Code sample for use of Stamen tiles with Leaflet becomes:

// replace "toner" here with "terrain" or "watercolor"
  var layer = L.stamenTileLayer("toner");
  var map = L.map("element_id", {
    center: L.latLng(37.7, -122.4),
    zoom: 12
  });
  map.addLayer(layer);
Merge pull request #47 from aergul/patch-1

Factory function added for consistency with Leaflet conventions