WIP: Module cleanup

Still WIP, and completely un-tested ;)

* [ ] Make methods similar to RESTful
* [ ] Remove preprocess -- it's not up for this module to change the field value. What was the original use case?
* [ ] Pass more arguments to the validate callbacks, and allow passing sub_property (e.g. $wrapper->body->value) instead of the whole value array.

@RoySegall @mateu-aguilo-bosch
Next is some cleanup for the name property/ field_name -- although I'm not sure which one is the correct one
Validate a field with multiple cardinality is not empty.