* temporarily unlinked a lib for xstream to prevent errors. Xstream can't be used for now!
* corrected the existing ListView in activity_main.xml
* added the photoList attribute to the MainActivity
* implemented the possibility to add photos to the photoList (and further to the listView)
* created a dummy method for initial data loading (loadPhotos)
* altered the toString method of the photo class, since it's called to determine the string which is shown in the listview.