Add  $max operation

Add $max operator to casbah.

The $max operator updates the value of the field to a specified value if the specified value is greater than the current value of the field. If the field does not exists, the $max operator sets the field to the specified value. The $max operator can compare values of different types, using the BSON comparison order.

http://docs.mongodb.org/manual/reference/operator/update/max/