Extend Leaflet's TileLayer, which has a fixed template URL string, to
accept a user-defined function urlFun instead, which takes an object
as input and must return a ready-to-use URL string.
Tested with Leaflet v0.7.2 as of 2015-03-03
Live example: http://alpstein.github.io/leaflet-tilelayer_urlfun/
https://github.com/ismyrnow/Leaflet.functionaltilelayer has promise
support, but duplicates the code of the TileLayer.getTileUrl logic.