Feature/output cache helper

Helper functions for the Core Output class to allow development of cache management controllers.  The functions are fairly straight forward, and are primary targeted towards developers looking to be able to clear specific cached content without completely clearing the cache directory.

I used the 'output_cache' name to identify the functions, since 'cache' may be confused with the 'Cache' class.  Are there any thoughts for alternative names? (I considered using 'ocache' instead)