added google cloud messaging introduction and quick start
Merge branch 'master' of https://github.com/zendframework/zf2-documentation into feature/zendservice-googlecloudmessaging
Renamed GoogleCloudMessaging to Google\Gcm
Resolved issue where there was still a reference to googlecloudmessaging instead of google.gcm
Merge pull request #508 from mwillbanks/feature/zendservice-googlecloudmessaging

ZendService\Google