Express: add view engine support (view without extension).

This PR is an attempt to fix #214 (Express `View` compatibility).

To render views without extension, we must set `view engine` setting: 

```js
app.set('view engine', 'html');
```

Then it should work: 

```js
app.get('/some-route', function(req, res) {
  res.render('some-view');
});
```

Tests are under `tests/express` directory.

There are some new `devDependencies`:

* [express 4.x](http://expressjs.com/)
* [supertest](https://github.com/visionmedia/supertest)