added new method removeInterceptor to remove one specific mock

This allows removing a specific interceptor for a url. It's useful when you have a list of common interceptors but one test requires one of them to behave differently.

Examples:
```js
nock.removeInterceptor({
  hostname : 'localhost',
  path : '/mockedResource'
});
```

```js
nock.removeInterceptor({
  hostname : 'localhost',
  path : '/login'
  method: 'POST'
  proto : 'https'
});
```