Issue 2: serve cloud tiles.

Most of the stuff in this diff is me merging master into the new branch I created to work on this issue (https://github.com/zoomhub/zoomhub/issues/2). My actual changes are in this commit: https://github.com/zoomhub/zoomhub/commit/1454c16a89ed9746e7dcad1c8c21a3bacfb2f4c0.

Although looking at the history, I may have somehow picked up a bunch of stuff from the migrate-content branch as well.

Someone who isn't so new to git may want to give me some advice here, or merge only that commit that I linked to above.

Oops, looks like this still needs updating.

Btw, I was suggesting that we may not want the config name to say "CDN", as a CDN doesn't need to be assumed (and in fact, may not be there for development environments).