Merge branch 'develop' of github.com:hpcugent/easybuild-framework into develop
hold jobs until all jobs are submitted, then release holds
keep track of holds on jobs, only set hold if job has no dependencies
add support for submitting job with (user) hold, use pbs.ATTR_x constants instead of using hardcoded strings
add missing self
add debug info for releasing holds
make sure hold used in submit() is registered, fix location of info log msg
fix defining list of known hold types, should be done after import pbs
correct function signatures of set_hold, release_hold, don't assume pbs module is always available
Merge pull request #751 from boegel/hold_jobs

hold jobs until all jobs are submitted, then release holds