Release 0.11.3

Bug-fix release. Changes are:

-  `JobMonitor` is now a context manager so that all jobs get killed when an exception occurs.
-  All jobs are now killed if a single job encounters an exception.
-  Jobs no longer pass back exceptions as strings, where they go totally unnoticed.
-  Cleaned up debug output a bit.
-  Much prettier tracebacks for job exceptions.