Add hook to delay `start()`

For Ext.js, ember, and perhaps other applications, the test runner could use a direct API for delaying the test suite from running. Here's my current work-around:

``` js
window.__testacular__.loaded = function(){};
someApp.on('init', function() {
window.__testacular__.start();
});
```

And then add it to the files array in the test config. Haven't had a chance to think about the API for that yet.