Delay check if Locator is mounted at runtime to allow flexibility in how express-view extends the app instance.


This fix allows the following in app.js

```
var app = express(),
      expview = require('express-view'),
      expyui = require('express-yui');

expview.extend(app);
expyui.extend(app);

app.set(xxx, yyy);

// << configure locator here >>
// extending expview should not depend on Locator being already mounted.
```

The current version requires that Locator be mounted prior to `expview.extend(app);` is called.