Excise tile getters?

@hadley suggested putting the tile retrieval functions in their own package; he even started one a few years back: __[rastermap](https://github.com/hadley/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.