Minor bug fixes

-RenderNodeMain.py: whenever a child got killed, it would neglect to
update the "host" field in the job after sending it back to the farm. It
now sets the "host" field to None to indicate that there is no render
node associated with the job.
-FarmView.py: The table of status codes was missing a key/value for 'S',
which means Started. FarmView now properly displays a count of started
jobs.