Adding in memory cache for images, copying the behaviour of [UIImage ima...

As promised I added a simple NSCache for the images already loaded so to reduce the footprint of requests that happens multiple times. 
I made changes on the class that are meant to stay there, but I also modified the controller to add a button in it that triggers a test and measure performances (profile allocation included in folder _etc).
If you are convinced by that you can then remove the changes on the Controller and all the stack traces from the code base.