Implement a custom image caching/downloading system

[RobertLK]

Context

• Images that are downloaded once, shouldn't need to be requested again
• The cache must obey to cache policies sent by the remote server (read https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching)

Specs

• Can hold up 10k elements (on disk)
• when the 10001 element wants to be cached, the image that was used the least should be uncached (aka if 9999 were requested twice and 1 image was requested once, the latter should be uncached). If two (or more) elements have the same lowest count, uncache any of them.
• public interface should be :

    interface MySuperCache {
         void get(String imageUrlString, CacheCallback completionBlock);
     }

    interface CacheCallback {
        void onImageRetrieved(Bitmap image);
    }

• Class(es) should be testable and tested
• Don't use HttpResponseCache or LruCache
• Don't use external frameworks (we know they exist)
• Should be reusable (this is why the interface is generic)