Sugar sintaxe for times method: "once", "twice", and "thrice"

Sugar sintaxe for times method

```js
nock('http://zombo.com').get('/').once().reply(200, 'Ok');
nock('http://zombo.com').get('/').twice().reply(200, 'Ok');
nock('http://zombo.com').get('/').thrice().reply(200, 'Ok');
```