Split up this package

There is not much shared code between the managers and most of us only use a single workload/cluster manager so it is difficult to review PRs.

### Finished:

- [x] Slurm (via `srun`): Done: [JuliaParallel/SlurmClusterManager.jl](https://github.com/JuliaParallel/SlurmClusterManager.jl)
- [x] Load Sharing Facility (LSF) (via `bsub`): Done: [JuliaParallel/LSFClusterManager.jl](https://github.com/JuliaParallel/LSFClusterManager.jl)
- [x] `ElasticManager`: Done: https://github.com/JuliaParallel/ElasticClusterManager.jl

### Currently in progress:

- [ ] HTCondor (via `condor_submit`) : In progress here: https://github.com/JuliaParallel/HTCondorClusterManager.jl

### Not yet started:

- [ ] Scyld (via `bpsh`): not started yet. Currently still lives in this package. We are looking to recruit a maintainer.
- [ ] Sun Grid Engine (SGE) (via `qsub` and/or `qrsh`): not started yet. Currently still lives in this package. We are looking to recruit a maintainer.
- [ ] Portable Batch System (PBS) (via `qsub`): not started yet. Currently still lives in this package. We are looking to recruit a maintainer.
- [ ] `LocalAffinityManager` (for pinning local workers to specific cores): not started yet. Currently still lives in this package. We are looking to recruit a maintainer.