Move native_parser option to Mongo's DB contructor

The mongo docs for Server (http://mongodb.github.io/node-mongodb-native/api-generated/server.html) do not list native_parser as an option, but the docs for Db (http://mongodb.github.io/node-mongodb-native/api-generated/db.html) do.

This change moves the native_parser option to the class that is documented to support it.