Excise tile getters?

[dkahle]

@hadley suggested putting the tile retrieval functions in their own package; he even started one a few years back: rastermap.

What do we see to be the advantages of that? It'd help with the size of ggmap, for sure, and you could think of an analogous package for geocoding and the other services. On the other hand, it may create a specialistic chaos of little packages, unless you group them around different company's API's (e.g. all the Google ones go in a single package; I'd imagine there's something out there already like that). Thoughts? We would also need to consider the role of ggmap itself in that context.

[hadley]

I think main advantage is decoupling the mappers from ggmap. But that could also be achieved by including rastermap inside of ggmap.