Support Map, List and Bool type

DynamoDB now supports to store entire JSON-formatted documents as single items.
http://aws.amazon.com/jp/blogs/aws/dynamodb-update-json-and-more/
In this update, following new attribute types have been added:
 * Map
 * List
 * Boolean
 * Null

This pull request adds a feature to support Map, List and Boolean type of attribute.
In Map and List attribute, it currently does not validate its content format.