Add support some data types in DynamoDB

Add support `List`, `Map`, `Null`, `Boolean` data type.
It's not obvious to me if this will break any existing users. What if I have a table with entries that contain bools? What if I'm writing bools into existing tables? Will this continue to work? What if I also use a static language like Java which expects these values in my tables to be numbers and I update boto with this change? I'm just trying to be sure that we know the effects of this seemingly minor change.