add thumbnail management: they are generated and the end of a job
the most recent one is displayed
if the shot is reset, thumnbails  are deleted
print the blender output during job working instead at the job end
update gitignore
remove useless calls to system_path
remove all system_path calls (python seems to handle path very well); add gobal var RENDER_PATH
add Pillow to requirements
Merge pull request #45 from msimoncz/thumbnail

Thumbnail