Support initializing with image URLs

[ZevEisenberg]

In some cases, you might have URLs for images (and possibly thumbnails), and it would make sense for RIGImageGallery to manage loading those images internally. If we don't want to own the actual URL requests, we could expose a block or delegate method so that the consumer provides some synchronous image loading code that RIGImageGallery calls asynchronously on their behalf. Coupled with this, RIGImageGallery should support a default per-page loading spinner and a customization point to provide a loading view, which optionally can show percentage progress instead of just "loading".

[ateliercw]

+1 for needing to find a good way to inject a loading indicator view / progress tracker view

I don't want to provide any sort of actual URL loading in the library, because there are better solutions out there (AlamoFireImage, SDWebImage), but writing sub-specs that integrate directly with those libraries and handle progress reporting / loading URLS seems like a good addition.