Changes to barcode unenroll script from Spark
Tweaks to unenroll script

- Use argparse to take program ID as command line argument
- Prompt to delete registrations (so it doesn't have to be run within a
  shell)
- Allow --per-hour argument to decide whether students should be removed
  from all of their classes, or only ones starting in the next hour
Rename barcode_unenroll.py to unenroll_students.py
makes it possible to run the unenroll students script as ./unenroll_students.py
Merge pull request #1175 from learning-unlimited/spark-barcode-unenroll

Changes to barcode unenroll script from Spark