Allow handlers to control which runner plugins they will load.
Allow runners plugins to accept params, provide some simple load-time validation logic for params.
Make job terminal state logic somewhat configurable via runner params, update
DRMAA runner plugin changes for configurable terminal state handling.  Also
allow the drmaa plugin to read the drmaa library path from job_conf.xml rather
than the $DRMAA_LIBRARY_PATH environment variable.
Add a Slurm runner (subclassed from the drmaa runner), adds some logic for
determining/reporting the reason of job failure and requeuing jobs that failed
due to node failure.
Sample job_conf.xml for new features introduced.
Merged in natefoo/galaxy-central (pull request #290)

Configurable terminal job state, Slurm enhancements