The present implementation coerces sequences it can't determine the type of to a string set and calls toString on it's members. This simple form of improvised serialization can work in most cases, but tends to be an impediment in the case of nested object graphs.

This patch is conceptually similar to #99, but also unpacks the values to their AWScala equivalents aws.dynamodb.model.AttributeValue => AttributeValue on fetch. This is meant as a step towards #110, but a real fix for that would require removing the JMap signature of the m property, for example. Fixing that would seem to unblock true native document support, as requested in #103 and #66.

Would appreciate feedback on the approach here as parts feel a bit clumsy.