Parallel jobs start script for SLURM clustering

A Script for SLURM (https://computing.llnl.gov/linux/slurm/) clustering support for Qiime. 

Note: The start_parallel_jobs_slurm.py has only minor changes compared to start_parallel_jobs_sc.py, however from Qiime architecture it seems that currently sharing of start_parallel_jobs* scripts is not supported. 

The script has been tested in a cluster using SLURM by running a Qiime analysis pipeline. The pipeline run completed without errors.