Enable support for MetadataType in Dynamo DB document Types

The current implemenation does not support the
System.ComponentModel.DataAnnotations.MetadataType attribute on DynamoDB
documents.  This attribute is often setup on entities that have
previously been saved to a traditional DB.  This changes enables lookup
on the metadata class.  The lookup is performed in a manner that is both forwards (to 4.0) and backwards compatible (with 3.5 Client Profile).