#79 Include x-headers in response hash

[davetapley]

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.