Adds validate_doc_update function support

Using the pouchdb-validation plug-in. Passes ``npm run pouchdb-test``. Currently just assumes admin party, so only useful for document validation, not user validation. But that can easily be added by supplying the right argument option in the db.put/post/bulkDocs/remove functions when session/security support is added. One downside: every write to the database takes a bit longer because every write needs to check if there are any design documents (which contain the validate_doc_update functions) available. Probably acceptable (CouchDB needs to do it too after all), but worth mentioning.