Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work described in this article was supported by a grant funded by the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. PolyU 5461/11H, 546111) and internal grants funded by the Hong Kong Polytechnic University (project nos. 1-ZV3U, A-PK54).