URI schema resolver

[jwueller]

Currently, external schemata are ignored. Example:

{ "$ref": "http://example.org/schema/definitions.json#/definitions/uuid" }

I would propose something along the lines of this:

var validate = validator(schema, {
    schemaUriResolver: function(uri, callback) {
         // uri is "http://example.org/schema/definitions.json", excluding the fragment
         http.get(uri, function(res) {
             callback(JSON.parse(res.body));
         });
    }
});

This would obviously require some restructuring of the code, since it is not async-compatible at the moment.

Alternatively, it would be great if we could accept top-level schemata instead of duplicating all sub-schema paths when providing those up-front.

Any thoughts?

[chrahunt]

The implementation in #69 doesn't change the current behavior here which I believe would still break your example. Wasn't sure if it would be proper to remove that line since I'm not sure why it is there.