Improve CloudPush documentation

[jonek]

The documentation of the CloudPush client service is missing a description of the default client behaviour:
http://developer.eclipsesource.com/tabris/docs/1.4/working/client-services/cloud-push/

The current docs can be misleading because they do not offer hints on how to create notifications in Tabris remote and that is a crucial part of signaling incoming cloud messages to the user. This behavior is part of our default implementation of CloudPush in Tabris.

Add some description about the notifications that are shown and how to customize that notifications.
Add some screenshots.