Added ALAssetsLibrary support

These two commits add ALAssetsLibrary support for loading local assets via SDWebImage.

I've integrated the support into SDImageCache for a much smaller footprint than adding this support via a separate set of NSOperations and all the overhead that comes along with that code path.

Note: This currently only caches local assets in the in-memory cache. The reasoning behind this was:

1) These images already exist on disk and can be retrieved via the ALAssetsLibrary
2) save CPU and IO cycles