Acknowledgments
Professor Xiaogang Hao gratefully acknowledges support provided by the Chinese Scholarship Council for a research leave at the University of Western Ontario. This work was financially supported by the Shanxi Scholarship Council of China (No. 2013051), the International Joint Research Project of Shanxi Province in collaboration with University of Waterloo (No. 2011081028) and the Taiyuan University of Technology Fund (No. tyut-rc201261a).