Preloader system with glide extensions

[elihart]

Updated version of #593

This creates a generic preloading system, with integration into EpoxyRecyclerView.

It also adds an optional module for easy prefetching with glide.

Usage is as simple as

recycler_view.addGlidePreloader(
            Glide.with(this),
            preloader = glidePreloader { requestManager, model: ImageModel_, _ ->
                requestManager.loadImage(model.imageUrl)
            }
        )

Once this is merged I'll push a new release with it and add a usage guide to the wiki.

If anyone is interested in adding support for other image libraries, like Picasso, feel free to open a PR and copy the glide implementation here. It is pretty straightforward.

[elihart]

There is an issue where running the annotation processor in tests fails fails to properly compile because of issues with the new kotlin code in epoxy-adapter. Can't merge until that is figured out