Pbs backend + bug fix

The bug-fix is a one-line change.

The PBS backend is a big commit which includes tests and all.
The test workflow(s) are a bit fragile: some failure trigger the clean-up too soon. But it is a testing problem; they should not block the PBS backend.

EDIT: grammar

https://github.com/hammerlab/ketrew/issues/24

Created is what the user/client has to define, running is what necessary to keep track of the job while it is running (or after)