Use Pillow instead of PIL

PIL is unofficially not maintained anymore and its installation via pip 1.5 or
later now requires to pass --allow-external PIL --allow-unverified PIL
The proposal of this commit is to use Pillow as a replacement.
Replace PIL by Pillow in misc. logging output
Merge pull request #2090 from sbesson/rebased/develop/pillow

Use Pillow instead of PIL (rebased onto develop)