This will use crontab to automatically restart the machine and resume
the script when the machine crashes. An additional option, -k, is provided
to cancel this operation.

If the retry flag is specified, failed experiments will be re-run after all
other experiments have run. They can be re-run at most 5 times.

This commit required a refactoring of run_exps.py to clean up the main
experiment running loop.

Thanks to bcw and gelliott for debugging and ideas.

* Print out experiment number and total experiments when starting experiments.
* Only sleep and re-release tasks if tasks are waiting to release.
* Fail experiment with verbose messages if any tasks fail before becoming ready
to release.
* When waiting for tasks to become ready for release, reset the waiting time
whenever a new task (or task(s)) become ready.
* Start regular tracers BEFORE the plugin switch to log data from the switch.
* Check the number of running tasks AFTER trying to switch the linux scheduler.
This gives plugin deactivate code the opportunity to kill these tasks.
* If an invalid executable is specified in the schedule file, fail before
attempting to run the experiment and print out the problem.
* Propogate exceptions up from experiment failures instead of creating
ExperimentFailed exceptions.

This commit also made clock-frequency automatically ignored by parse_exps.py.
The value of this would change by +- a Mhz between experiments, ruining graphs.