Custom image cache search paths

This feature lets you define custom paths for SDWebImage to search for previously-cached-images.

This is particularly useful if you are bundling images with your app that have been cached by SDWebImage. (ie. if you are 'seeding' your app with a core-data file that contains a lot of URL's to images and would like to also seed those images without having to copy every one of them over)

For example, you can tell SDImageCache to add '[[NSBundle mainBundle] resourcePath]' as a custom path, so that the main bundle will be queried for cached images. 

This prevents the need for you to copy pre-cached images over to the caches/ImageCache folder that SDImageCache normally checks for.

The custom paths are read-only.