Response paging

Added response pagination.

The new response paging plugin wraps the response in a delegate object that responds to `#last_page?`, `#next_page?` and `#next_page`.

```ruby
resp = iam.list_users
resp.last_page? #=> false
resp.next_page? #=> true

# send a new request for the next page of results
resp = resp.next_page
```

You can still access all of the normal properties on the response.