Avoid leaking file descriptors

This change prevents img2pdf from opening *all* input files at once,
which means it now works with thousands of input files.
Convert unrecognized colorspaces to RGB

Instead of crashing on an unrecognized colorspace, we now do
imgdata.convert('RGB').
Use "with" to open and close input files
Merge pull request #18 from DarwinAwardWinner/master

Avoid leaking file descriptors and convert unrecognized colorspaces to RGB