create cache clearing mechanism

I have a need to allow users to select from a changing set of data.  If you're amenable to the spirit of this change let me know and I'll add tests and rebuild the required bits.

...and if I'm missing some obvious way to achieve my needs without nuking the cache object please let me know :) (and sorry for wasting your time)

***thank you!***

*great, well-constructed project btw!*