DynamoDB Document/Item (Un)Marshaler

Added a Marshaler helper object to the DynamoDB namespace that can handle maps and lists.
Specifically, what use cases did you have in mind for this?

The reasoning for this class I thought was to make it easier to represent JSON like data in DynamoDB documents. I'm not sure that this class was designed for setting single values where you might pass them into something like a create table operation. I think you'd want to be much more explicit in those cases and provide the DynamoDB types directly.