Refactored packages: command & operation.

@Jeff, please take a look. I've decided not to with command pattern for org.mongodb.command.* and left it as it is. Wrapping command results looks fine for me.

Also I've update things related to FindAndModify. I believe that having single class that corresponds directly to http://docs.mongodb.org/manual/reference/command/findAndModify/ is cleaner. 
MongoOperation is annotation now just for informative purposes.