Custom image cache search paths

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.
Refactor GIF support

Remove GIF specific code from main source, isolate it in a UIImage category to ease future addition of format support
Add WebP format support (fix #410)

How awesome is that?! =)
Merge pull request #2 from rs/master

merge author update