lib: implement the PBS backend + integration test

There is a new module `Ketrew_pbs` which implements LONG_RUNNING with
PBS/Torque. This was developed in conjunction with integration tests:
the test uses Ketrew to build a vagrant VM with Torque installed; then
the PBS setup is finalized, and then 2 PBS jobs are run (one of them
always fails, it is supposed to).
lib: fix bug in `Engine.add_targets`

When checking targets for redundancies Ketrew was testing only against
the targets in the database; not the ones within the request itself.