Add method to validate/compare mapped indexes with existing ones

[evanchooly]

This would be a peer to ensureIndexes() that would take the mapped indexes and compare them with what's in the database. Possible options to the validator would be to log the differences, fail on diff, output the shell commands to create the missing/different indexes. Also, it should be configurable to allow ignoring indexes defined in the database that aren't mapped via morphia.