Package reorganisation

This release includes some package reorganisations. The following package structures have changed:

// Before:
use gcm::message::Message;
use gcm::message::response::{GcmError, GcmResponse};
use gcm::notification::{Notification, NotificationBuilder};

// After:
use gcm::{Message, Notification, NotificationBuilder, GcmError, GcmResponse};

// or more simply:
use gcm::*;