Add imageTileLayer

[manthey]

Currently we have tileLayer which we subclass as osmLayer for image tiles. This proposes that we add imageTileLayer, and have osmLayer be a subclass of imageTileLayer. imageTileLayer would not set the url, attribution, and tileOffset.

This might also be an opportunity to address the old issue #60, where we could have an object that would contain known tile sets for the osmLayer (which, if any aren't from OpenStreetMap, would continue with the inaccurate naming).