Emit grunt event when server has started

To provide support for writing tasks like 'jasmine-server':

    grunt.registerTask('jasmine-server', 'start web server for jasmine tests in browser', function() {
        grunt.task.run('jasmine:tests:build');

        grunt.event.once('connect.tests.listening', function(host, port) {
            var specRunnerUrl = 'http://' + host + ':' + port + '/_SpecRunner.html';
            grunt.log.writeln('Jasmine specs available at: ' + specRunnerUrl);
            require('open')(specRunnerUrl);
        });

        grunt.task.run('connect:tests:keepalive');
    });