Implementing in-app billing: Functionality
 - add products in developers console (merchang account obligatory)
 - create interface to offer products to user (e.g. fragment_in_app.xml)
 - decide to use local or dislocated store of purchase informations
(e.g. PreferenceManager.java)
 - add buying and billing functionality (InAppBillingFragment.java)
 - connect with main activity if neccessary
Test in-app billing
 - ensure to have test account setuped in developer console
 - or use predefined test skus (btw. not working very well)