Changes PIL to Pillow

This changes our official requirements from PIL to pillow. It seems to work
well, as Pillow aims to provide a smooth transition, and it obviously doesn't
mean anyone is supposed to uninstall PIL and get Pillow right away.

More info at: https://github.com/python-imaging/Pillow
Moves test_getEXIFData from required to optional

As exifread isn't a requirement but rather something optional for whoever needs
it, tests using it shouldn't be inside tests.py. Travis should be happy now!