Added support for access to the client response.

[mando]

The client object now has access to the response object, which contains the following:

• body - raw response body
• headers - filtered header list
• status - status code
• method - HTTP verb
• request_uri - uri of the last action

By default, the only response headers that will be returned are the Twitter response headers:

• x-ratelimit-limit
• x-ratelimit-remaining
• x-ratelimit-reset

To add additional response headers, simply send them as an option to the client on creation:

c = Grackle::Client.new(:response_headers=>['X-My-Awesome-Header'])