Implement separate batchjobs future constructors

[HenrikBengtsson]

Implement separate batchjobs future constructors, e.g.

## Generic (fully compatible with BatchJobs)
plan(batchjobs) ## for .BatchJobs.R
plan(batchjobs, ".BatchJobs.R")

## Template-based setups (with default template files)
plan(batchjobs_torque)
plan(batchjobs_torque, template="torque.tmpl")
plan(batchjobs_slurm, template="slurm.tmpl")
plan(batchjobs_sge, template="sge.tmpl")
plan(batchjobs_lsf, template="lsf.tmpl")

## Built-in setups
plan(batchjobs_interactive)
plan(batchjobs_local)
plan(batchjobs_multicore, workers=4)
plan(batchjobs_ssh, workers=nodes)

(Also, since plan(batchjobs) is basically the only real usage of future.BatchJobs, it won't change much of the existing code. The other types of BatchJobs futures are mostly there for proof-of-concept since they are already supported by the core futures in the future package.)

This is actually much more in line with what's in the future package, cf.

plan(transparent)
plan(eager)
plan(multicore, workers=4)
plan(cluster, workers=nodes)

[HenrikBengtsson]

This far we have:

plan(batchjobs_local)
plan(batchjobs_interactive)
plan(batchjobs_multicore)
plan(batchjobs_multicore, workers=n)
plan(batchjobs_conf)  ## Replaces backend(".BatchJobs.R")
plan(batchjobs_conf, pathnames=pathnames)   ## Replaces backend(pathname[1])