Deleting unused Commands
Replaced remaining use of Create command with command operation; deleted Create command; moved createCollection tests from Old legacy test into slightly less old JUnit test
Replaced Distinct command with DistinctOperation; moved distinct tests from DBCollectionOldTest to DBCollectionFunctionalSpecification and beefed them up a bit (adding Hamcrest to do so);
Moved al the tests from GroupTest into DBCollectionFunctionalSpecification and groovy-fied them, because the Group command is going away.
Replaced Group command with GroupOperation.  Removed deprecated group method on DBCollection that takes a DBObject.
Updated AggregateOperation to use the new read preference selector helper.
Added class-level JavaDoc
Added links to the mongodb.org documentation using the custom taglet
Removed warning
Moved Distinct and Group operations into correct package/directory