useless files
Add validate_doc_read property to a design document.

Like validate_doc_update, this function validate if the document can be
read by the curreny user.

ex:

    function(doc, userCtx) {
        if ((typeof doc.name !== 'undefined') && (doc.name !=
userCtx.name)) {
            throw({unauthorized: doc.name + ' cannnot read ' +
doc._id});
        }
    }

will allow the current user to only read the documents where the
properties name is the name of the user.

Note: admins can always read the documents.