Provide ability to fetch non-image tiles

As MapBox, OSM, and others are adopting Vector tile formats, it is useful to provide the ability to obtain the tiles through this simple API and not assume they are images to be stitched together. This changeset includes a minor refactoring of the existing code such that a new method is exposed which returns the TileQueue post-fetch by including a base method in the Provider API which can be overridden when non-image tiles will be returned.

I pondered renaming the internal storage and access api (imgs / images()), or diverting the data into an alternate variable/function. However, I wasn't sure if that was necessary since python won't care what data it's obtaining anyhow. would love some feedback on this though.