Prevent shell injection in load_djpeg
Tests for _save_netpbm, _save_cjpeg and load_djpeg
Change most uses of os.system to use subprocess

The only places left that use os.system are in ImageShow and setup.py
Add libjpeg-turbo-progs to travis install
Better error checking in _save_netpbm
Skip tests if external commands aren't found
Removing previous approach
Suppress stderr from ppmquant and ppmtogif
Merge pull request #748 from wiredfool/djpeg

Prevent shell injection