changed appengine sdk version to 1.9.4

[mpoehler]

Update of the sdk version to 1.9.4

Setting the java version to 1.7 (since version 1.9.0 of the sdk java 7 is required)

There was an error that no default module exists in the application if deployed to a newly created, empty project. I have changed the name of the guestbook module to default which fixed that.

Added propergation of app version property from pom.xml to the different modules appengine-web.xmls.

Added a link in Guestbook to demonstrate the creation of links between modules. Work on the development server as well as in Production.