Created slurm launcher for workflow plugins

This hasn't been well tested but it should work without too many issues. Of particular note are the environment variables that we're referencing. Those commands have the capability to fail with no error and need to be watched closely.
Fixed misnamed variable causing crash.

_sbatch_args was incorrectly called _slurm_args in one of the if
statements in slurm.py causing a crash. Added in a load .slurm to
__init__ as development env had plugins in PYTHONPATH. Moved slurm
plugin base to base.py instead of slurm.py
Added support for sbatch args to slurm module.
Added mandatory args for SLURM, fixed regex for batch job number

Added in several defaults for mandatory arguments to sbatch. Also the
regex that finds the job number was malformed and was returning 'None'
for all batch jobs. That's been fixed and is now returning jobs
correctly.
Merge branch 'master' of https://github.com/ChadCumba/nipype