Added amd support as per https://github.com/umdjs/umd/blob/master/amdWeb.js
Added amd support as per https://github.com/umdjs/umd/blob/master/amdWeb.js

tomhughes/leaflet-providers,smileliaohua/leaflet-providers,andreaswc/leaflet-providers,leaflet-extras/leaflet-providers,kyroskoh/leaflet-providers,andrewmartini/leaflet-providers,smileliaohua/leaflet-providers,leaflet-extras/leaflet-providers,andrewmartini/leaflet-providers,andreaswc/leaflet-providers,tomhughes/leaflet-providers,ximex/leaflet-providers,ximex/leaflet-providers,drzhouq/drzhouq.github.io,kyroskoh/leaflet-providers,drzhouq/drzhouq.github.io