Improve agent robustness and cleanup

Improved the page timeout handler to prevent race conditions and
  allow more time for posting results.
Improved the getwork/workdone handler to catch I/O errors and
  recover correctly.
Added "--killall 1" support to run an agent as a "device" user, e.g.:
    sudo -u deviceX ./wptdriver --killall 1 ...
  and `killall` zombie processes between runs.
Abort wd_server if PAC is invalid
Fixed pcap unlink
Converted process_utils callbacks to schedules
Only send video/pcap to agent if file exists
Made process_utils "app" arg required.
Added scheduleKillTrees
Merge pull request #105 from wrightt/master

Improve agent robustness and cleanup