Clear all photos from cache

[mdstone06]

I would like to propose an option to clear the entire cache in the bichooser folder. Currently, there is the clearOldFiles() method, but this only deletes the oldest files to get the "cache" folder under 500MB total size. It would be nice to be able to clear everything in that folder, regardless of size/age.

As pointed out in another issue #183 there are multiple copies of my images saving to the bichooser folder, which are being picked up by the phone's photo library, which I very much want to avoid.