Custom item marshaling

This release adds ItemMarshaler and ItemUnmarshaler, which are interfaces allowing for total control of item marshaling. This is useful for handling dynamic items whose structure depends on its content. Thanks to @luma for submitting this in PR #126!

// ItemMarshaler is the interface implemented by objects that
// can marshal themselves into an Item (a map of strings
// to AttributeValues).
type ItemMarshaler interface {
	MarshalDynamoItem() (map[string]*dynamodb.AttributeValue, error)
}

// ItemUnmarshaler is the interface implemented by
// objects that can unmarshal an Item (a map of strings
//to AttributeValues) into themselves.
type ItemUnmarshaler interface {
	UnmarshalDynamoItem(item map[string]*dynamodb.AttributeValue) error
}