Add feature to clear exhibit lens object cache

[distobj]

As a result of the closing of issue #13, we identified the need to permit authors to explicitly clear the lens object cache associated with hosted exhibit data, which they would need to use when after editing their exhibit HTML.

[zepheiryan]

I haven't tested out the latest changes to see the failure mode you refer to, but I'm guessing the CORSify method needs to allow DELETE in order to interact with cache clearing from the browser.

[distobj]

Sorry, I meant to update this ticket when I pushed that commit...

Good point, but the testing I've been doing has been from the command line so CORS wasn't in play.

The failure I referred to was that .clear()-ing that cache doesn't have the desired result and I continued to receive session errors.