Implement a custom image caching/downloading system

[CharlesVu]

User Story

As a user, I would like to avoid redownloading a photo if I have already downloaded it.

Specifications

Implement an image cache system that will respect the following

• Download images on first image appearance.
• In case an image is already downloaded then it should be loaded from the local cache.
• Our local cache should be able to hold 10000 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, remove any of them.
• Don't use NSURLCache