Log kill_instruments with DEBUG_UNIX_CALLS
Use command fetch child and parent processes

* The parent is `sh xcrun instruments`.
* The child is the full instruments path.
Fix FIND_PIDS_CMD documentation
Ensure pids are sorted ascending

We want to kill the child, wait and then kill the parent
to avoid orphaning the instruments process.
Update pids_from_ps_output example
Merge pull request #87 from calabash/feature/avoid-orphan-instruments-processes

instruments process are becoming orphaned because the parent is killed before the child