Added support for ALAsset and PHAsset image retrieval and caching.

My main goal with this addition was to make my image retrieval pipeline as simple as possible (single entry point for retrieving all of my images, regardless of where they live). I have implemented ALAssetsLibrary support with a previous pull request (#571), but we decided it wasn't a clean enough approach. This time, I think the separation of concerns and overall minimal changes to the core of SDWebImage makes for a better solution. 

All of the Local Image Store code is done in a category on SDImageCache this time. Happy to discuss any implementation details. I wanted to put this out there to start a conversation around what else the SDWebImage community would like to see in an addition like this. I would love to see it get merged into master, though we should verify it's something others would find use in.