Bugfix for job cancellation (reported by Carsten in email)
Ended up recoding part of the communication pipe (use json more consistently)
Fix bpy data modifications where it shouldn't happen (as a bonus, thumbnailing is now done out of process)
thumbnails on the web interface (resized with imagemagick or compatible if present, fullsize otherwise)
Clicking on the "show" link shows the thumbnail associated with one frame. Clicking on a second one shows all frames in the range. Clicking on more frames extends the range. Clicking on a thumbnail (or on the show link of a visible thumbnail) hides all thumbnails.
Slave and Master options to delete files when closed (default True for slave, False for Master)
Web interface option to remove files (on master) when deleting a job (or all jobs)
Web interface button to pause a job
