Add API for setting named tile sets

[jeffbaumes]

Like openstreetmaps, mapquest, etc.

[aashish24]

@jeffbaumes @manthey can we close this issue now?

[manthey]

Currently, you have to set the tile url as the whole string (e.g., http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png). I think this was proposing setting a dictionary of these values: tiles = {osm: 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', 'stamen-toner-lite': 'http://tile.stamen.com/toner-lite/{z}/{x}/{y}.png'} and then supporting setting the tile url as a single word: layer.url('osm').

This would make it convenient to specify a few tile sources quickly; we default to OSM, but if we wanted to have a short list of common tile servers, this would facilitate it.

[aashish24]

I see I thought it was this http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png but perhaps @jeffbaumes can confirm and most likely you are correct.