Support the signed header.

Now we can enable the signed header by passing the `client_ips` keyword argument
to the Instagram.Client constructor.

    c = client.InstagramAPI(
            access_token=access_token,
            client_ips="1.2.3.4",
            client_secret="CS"
        )
    c.follow_user(123) # => call API with X-Insta-Forwarded-For signature.

If you do not set the client_ips nor client_secret, X-Insta-Forwarded-For is not sent.