More launchers

crew.cluster 0.1.0

• Submit workers asynchronously (#2).
• Use system2() instead of processx to submit workers (#2).
• Add a verbose argument to the SGE launcher to optionally print system2() stdout and stderr.
• Implement SLURM and PBS/TORQUE launchers (#1, #5, #6).
• Create an abstract cluster launcher class to combine common elements of cluster launchers.
• Add a script_directory argument to customize the location of job scripts (#3, @mglev1n).
• Implement an LSF launcher (#4, @mglev1n).