Validate publications with json schema

[rjzondervan]

This adds the functionality to validate the data-field in a publication with JSON-schema definitions built in the publication type.

If there are no validation errors the field validation remains empty, while it will return data if there are validation errors. These errors follow the following form:

{
    "/": ["the required properties (date) are missing"],
    "/date": ["The data must match the 'date' format"]
}

Please note that if there is an error on root level (like a missing parameter), the validator will not progress. (That means that the object above can never happen this way, so that object is just for illustrative purposes).