Release 0.12.2

Just fixed a couple minor issues.

-  `exception` is now the properly set as the cause of death when a job encounters an exception.
-  Fixed a potential memory leak in the `qmaster` process caused by not cleaning up job info as recommended in the [DRMAA Python documentation](http://drmaa-python.readthedocs.org/en/latest/tutorials.html#waiting-for-a-job).
-  Changed default `session_id` in `JobMonitor` to `None` to be more Pythonic, instead of -1 like it was before.