Finally, I want to show my most sincere gratitude to my supervisor Dr. Maciej Dunajski, for his high quality supervision, patience and attention, besides being so kind to revise this thesis. His encouragement and support during the most difficult times of my career is something I will never forget. Moreover, I would like to thank him for the various mathematical ideas he shared with me leading to the academic production we have had together. In particular, it is fair to attribute solely to him the ideas and proofs of Theorem 5.2 and Proposition 5.8.