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
Add (failing) test for #664
Possible fix for #664
flake8
Move to an elif clause of the if in line 778, as it's just another combination of mode,self.mode under the master transparency condition
remove redundant transparency check