Added support for marshalling a nil value.

This is one side of the nil issue. This code will allow a user to insert a nil value into a query so that cassandra will set the column to null.

The unmarshalling is more complex and so will come in a seperate PR.
Would it make sense to query for the `count` column as well, just to make sure that the marshalling is bi-directional, or would that over-extend the scope of this patch?