ResultCache: Cache the output generated by a functions

ResultCache is a class used to cache output of a function in disk files.

This class is good for improving the performance of scripts that output the results of slow database queries. It can be used for caching data generated from other sources besides database query results.

On first call of the class, the function is executed and the returned data to is stored in a cache file. On subsequent calss, the data is retrieved from the cache file.