Add support for caching

[jbaiter]

Currently Hymir does not do any caching on its own.
This is less than ideal if the images are fetched from a remote source, e.g. via HTTP or S3.
In this case, users should have the option to configure either an on-disk cache or simple LRU RAM caching.
This should be fairly simple to implement using something like EHcache.

[eroux]

excellent idea yes, we've implemented a cache with JCS on our hymir but EHcache looks good!