Add async/await examples to documentation

I didn't realize until after using this library for a few days that `supertest.Test` (the return value for `.get`, `.expect` etc) extends `Promise` after a few generations of inheritance.

This means that you can do the following:
```js
import request from 'supertest'
const response = await request.get('/').expect(200)
```

One key thing to note is that `.end` should NOT be called when using `async/await`