Rake task for purging deleted Monster entries

[gregmalcolm]

Need to write something that removes monster imports that need to be deleted.

Prerequisite: Need to implement #14 so we can tell all the records that are 100% updated by the batch job importing data from the Monster.

Rules:

• Only remove entries where the updaters are 100% EDDiscovery.
• Work from the child dependencies and work back, only pruning when there are no children.
• Only remove records that have not been updated in at least 6 hours.
• If the count of rows to change is above a sensible threshold abort then abort. We don't want to lose everything just because the Monster imports aren't importing for some reason.
• Provide some kind of scope on each model for pulling back purgable records so it's easy to check them over from the console without actually purging.