#79 Include x-headers in response hash

If this looks okay, then I can probably whip up some tests and docs too.
Here's how I'm using it:

```
    search = client.search_venues ll: [lat, lng].join(','), limit: limit, intent: 'checkin'
    $stdout.puts("measure#foursquare_venue_api_remaining=#{ search['x-headers']['x-ratelimit-remaining'] }")
```

I figure the probability of the key `x-headers` colliding with something in an actual response body is zero.