some fixes and new methods to get subscription statements work

fix friending, add method to look up ressources to find url of activity feed
add importResource method
change behaviour of feedToActivity method. Activities received via activity feeds will be imported
minor fix in testPingAction concerning usage of template
add new method getSubscriptions of a user
add subscription resources and change behaviour of existing methods. Add method that returns the uri of a useraccount by a given WebID
minor changes to template after checking w3c feed validator