Media Library Pre-renders some image sizes immediately after upload

[johnnyoffline]

The media library should at minimim pre-render image previews for the sizes in the media library.

This helps avoid the initial load time.

But expanding upon this idea, I think multiple pre-defined sizes should be rendered to work with the media library enhancements outlined in Ticket #20

[rickybanister]

If we implement cropping this ticket becomes irrelevant, it won't know how to save smaller sizes if they are to be cropped.

Also, it is primarily the Editor that pays the loading price for creating new images. When a slideshow is created, the Editor is directed back to the page with the newly resized images. An end user won't have to wait.