Add header info to response object

We return rate limiting data in the HTTP headers:

```
X-RateLimit-Limit: 5000
X-RateLimit-Remaining: 4050
X-RateLimit-Reset: 3300
```

Lets surface that data in the response object.

``` ruby
response = Clearbit::Person.find(email: 'alex@alexmaccaw.com')
response.headers["X-Next-Rate-Limit-Window"]
```

https://gist.github.com/croaky/d12b04fe940130834d12
