Add helper functions for most common caches

[vittoriom]

It would be nice to add some public functions or maybe one helper class with a couple of methods on it, to create the most common caches (e.g. Memory -> Disk -> Network)

This way one doesn't have to always write the boilerplate code for simple usages

[vittoriom]

• Include dataCache function for NSURL keys and NSData values
• Include imageCache function for NSURL keys and UIImage values
• what else? @andresbrun

[andresbrun]

I would feel pretty confortable by using these two so far ;)

[vittoriom]

Added dataCache() and imageCache() static functions to the CacheProvider class.
The implementation is not really production-ready but the flaws will be addressed by follow-up issues.