Remove simplePushEndpoint property from installation

full discussion here http://lists.jboss.org/pipermail/aerogear-dev/2014-February/006120.html
tldr :
The deviceToken will now contain the pushEndpointUrl , which made it redundant to keep the simplePushEndpoint property.
This affects :

• The registration endpoint (validation check on simplePushEndpoint)
• The model
• The sender / DAO (we have to retrieve the deviceToken instead of pushURLs)