angestrebter akademischer Grad

I want to express my gratitude to several persons, without whose help the com-pletion of this thesis would not have been possible. First, I am deeply indebted to my parents for supporting me all through my studies. The backup of my family often created new incitations to accomplish all the difficult periods in my studies. As well, I am very grateful to Dr. Christine Zulehner for taking the task of assisting me in my work. She suggested the present topic and offered practical support with several valuable ideas. Moreover I wish to thank Mag. Klara Zwickl for proof-reading and useful advices. Furthermore I want to thank all of my friends for their generous and patient moral support. Thank you all! Finally, I want to seize the opportunity to promote LATEX, the open source software with which I wrote this thesis