Add event handler to enable clearing the cache.

Why it's necessary to sometimes clear the cache:
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When you dynamically refresh a part of your page (i.e. via Ajax) that contains
images, and you then want to use imagesLoaded to determine whether these images
have finished loading, it will wrongly tell you that they are loaded because
it finds them in the cache.

It's therefore necessary to have a way of clearing the cache if one wants to
refresh sections of your page containing images (via Ajax).