Query

this allows you to pass `parse` function for each key, to allow less verbose testing.

so this:
```js
    it('should map screen calls correctly', function(done){
      var screen = {
        name: 'name',
        timestamp: new Date()
      };
      var event = JSON.stringify({
        time: screen.timestamp.getTime(),
        user_properties: {},
        event_type: 'Viewed name Screen',
        event_properties: {
          name: 'name'
        }
      });

      test
        .set(settings)
        .screen(screen)
        .query({
          api_key: settings.apiKey,
          event: event
        })
        .expects(200, done);
    });
```

becomes (note the `JSON.parse`.)

```js
    it('should map screen calls correctly', function(done){
      var json = test.fixture('screen-basic');
      test
        .set(settings)
        .screen(json.input)
        .query('api_key', settings.apiKey)
        .query('event', json.output, JSON.parse)
        .expects(200, done);
    });
```