Change unsigned INT16 to UINT16

Addresses issue #642
Fix 12-year-old FIXME
Some pep8 and pyflakes cleanup
Simple test for saving to PDF
Add some more pdf tests
Fix CMYK test
Temporarily remove failing P-mode PDF test
Add monochrome PDF test
Temporarily remove 'failing' 1-mode PDF test
Temporarily remove 'failing' 1-mode PDF test
Fix tostring()/tobytes() warning and reinstate test
Fix bytes/str and reinstate test
pep8 and pyflakes
Don't cover PyPy: it fails intermittently and is x5.8 slower (#640)
Keep PyPy first, it's still a little slower
Merge pull request #2 from hugovk/master

Tweak for #664
Merge pull request #665 from hugovk/transparency

Remove transparency resource after converting P->RGBA, Fixes #664
Update CHANGES.rst
Merge pull request #662 from hugovk/nocoverpypy

Don't run coverage on PyPy jobs
Merge pull request #643 from CounterPillow/datatype-fix

Change unsigned INT16 to UINT16