Add utility method for grabbing entire raw response

I needed to access the values for 'X-Ratelimit-Limit' and 'X-Ratelimit-Remaining' Instagram provides in its headers (See [Issue #63](https://github.com/Instagram/instagram-ruby-gem/issues/63)), so I created a very simple utility method that grabs the entire API response (including headers) instead of just the body. 

Now I can get the values I need like this:
```ruby
client = Instagram.client(:access_token => session[:access_token])
response = client.utils_raw_response
remaining = response.headers[:x_ratelimit_remaining]
limit = response.headers[:x_ratelimit_limit]
```