Add specific 32-bit float tiff format

We regularly use this format to store 32bit floats and I would like to see it handled by clean Pillow installations without having to add it on every system I use.
Test image: TIFF for OPEN_INFO = {(II, 0, 3, 1, (32,), ()): ("F", "F;32F")}
Merge pull request #614 from Hijackal/patch-1

Add specific 32-bit float tiff format