Slippymaps

This PR implements a generic interface to "slippymaps", the most common type of online map. 

I've converted `get_stamenmap()`, but `get_openmap()` and `get_cloudmade_map()` need some API changes. Rather than converting them, it might be better to start a fresh with a new API and new naming convention. I think it would sense to use `map` as the key verb, so you'd have `map_stamen()`, `map_osm()`, `map_cloudmade()` etc. Each function would have arguments `lon`, `lat`, and `zoom`, followed by any specific options for the provider (e.g. map style, or key), then output options `cache` and `quiet`.