Do not clobber Pillow if it is already installed.

Do not forcibly install PIL if Pillow is already installed. Until such
time that Pillow can report itself as providing PIL, all we can do now
is just try to detect it. Hopefully Pillow will not release a version
that allows:

import Image

As this will break detection.
Prefer Pillow>=2.0.0 instead of PIL.
Merge pull request #32 from rudeb0t/master

Fix setup.py clobbering already installed Pillow library.