restore alphabetical order to list of parse arguments
set PYTHONUNBUFFERED=1 in tox.ini
get_children() is deprecated; use children() instead
fork worker can run as root for testing
add username argument to create_job()
Ignore result of http callback task
raise exception on failure to setuid in fork worker
Catch failure to set uid
extend job user test to also test for group

It turned out that setting the user has the side-effect of also setting the
groups, so no additional deliverable source code is needed.
always reject requests to run as root
allow umask to be set
allow cwd to be set
change test name; whitespace
test exception when the job cwd does not exist
test exception when the job cwd is inaccessible
Check whether a directory was provided before attempting chdir
consistently state related function definitions
change terminology from `username` to just `user` to be consistent with api docs
change terminology from `cwd` to `working_directory` to be consistent with api docs
fix ordering of imports where they were not alphabetical