Allow serialized objects?

[dantleech]

Currently if you want to store an object as a property of a document, you need to map it as a child document

Doctrine ORM has the option to store ojects as serialized PHP, this is can be more desireable than mapping a child document for performance reasons.

So I would propose something like:

/**
 * @PHPCRODM\Field(type="php_object")
 */

(FTR Doctrine ORM uses object, not sure if that would be better here).

Alternatively we could make it possible for users to implement custom types (I don't think this is already possible?).