Adds SLURM launcher to pipeline plugins

This edits __init__.py, base.py, and adds slurm.py to pipeline/plugins. It adds ability to launch jobs on SLURM based cluster computing systems using the sbatch command. Usage is as follows:
workflow.run(plugin="SLURM", plugin_args={'sbatch_args':'<sbatch args here>'})

SLURMLikeBatchManagerBase extends SGELikeBatchManagerBase, but does not implement the MIT hack to wait for directories to become stable as that is not a concern on the system (Stampede at TACC) that it was built for.