Add `this.callback` alternative w/o error argument

Added a function `this.callbackWithoutError` in addition to `this.callback`. This will be used on a case-by-case basis when `topic` is asynchronous and result callback doesn't pass error as first parameter, instead passing the result (to be passed to a vow).

    var getFirstWord = function (str, callback) {
        callback(str.split(' ')[0]);
    };
    vows.describe('Example use-case').addBatch({
        'Calling getFirstWord with "hello world!"': {
            topic: function (router) {
                return router.route('hello world', this.callbackWithoutError);
            },
            'should result in "world"': function (result) {
                assert.equal('world', result);
            }
        }
    }).export(module);