Add support for metadata instance classes

[kevin-bates]

This pull request adds the ability for custom class implementations to be tied to metadata instances of the same schema.

Schemas that contain the system-level property metadata_class_name will have instances containing their metadata properties of that class. These instances, which are subclasses of Metadata (enforced), can then override methods post_load(), pre_save() and pre_delete() to allow the implementations to adjust their values, perform actions on behalf of the instance, etc.

The pre_save() method is called after validation, while post_load() is called prior to validation of the object against its schema. (Validation is not performed on deletes, so call order relative to validation is not applicable.)

Use of this feature is optional, but the metadata tests show how this can behave.

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[lresende]

LGTM. @kevin-bates could you please rebase this.