Add schema validation option using meta schema

[Bartvds]

Bundle or link to https://github.com/json-schema/json-schema/tree/master/draft-04 and verify the schema is valid.

[oliverguj]

I simply have a task "schemas", which validates all my schema files against the specs:

        schemas: {
            options: {
                root: 'http://json-schema.org/draft-04/schema#'
            },
            src: ['**/schema/**/*.json']
        },

I always run this task before all others.

[Bartvds]

Clever 👍