Adding size key validation to tests
Improving coverage of test suite

Includes:
* Removing unused
versatileimagefield.forms.VersatileImagePPOISelectField
* Simplifying VersatileImageMixIn.ppoi setter
* Validator clean-up (including removing support for tuples-as-strings
for PPOI values)
* Adding images with EXIF data
* Adding ‘verify against’ images (to ensure exif values are properly
processed)