Ipython-cluster-helper

- Add a special resource flag `-r mincores=n` which requires single core jobs to
use at least n cores. Useful for shared queues where we can only run multicore
jobs, and for sharing memory usage across multiple cores for programs with
spiky memory utilization like variant calling. Available on LSF and SLURM for
testing.
- Add hook to enable improved cleanup of controllers/engines from bcbio-nextgen.

- Make a cluster available after a single engine has registered. Avoids need to
wait for entire cluster to become available on busy clusters.
- Change default SLURM time limit to 1 day to avoid asking for excessive
resources. This can be overridden by passing `-r` resource with desired runtime.

- Respect RESOURCE_RESERVE_PER_SLOT for LSF. This causes resources to be specified
at the level of a core instead of a job

- Added ability to get direct view to a cluster.
- Use rusage for LSF jobs instead of --mem. This walls of the memory, preventing nodes
from being oversubscribed.