New command - empty_slots - defined to report currently empty slots

Usage:
   $> empty_slots [N]
      If N is provided, then will show all empty slots for examiner IS=N.
      Otherwise, will list all empty slots for all examiners

This command is more robust than the previous implementation.
For one, it looks at only a subset of the template during diff
analysis, thereby making it less suscpetible to changes in the
template outside of the portion being considered
Merge pull request #490 from abhinavc/master

New command - empty_slots - defined to report currently empty slots