There is an error in the designations for equal authorship contributions. The first two authors, Yan Yang and Wencheng Li, should be noted as contributing equally to this work.
Bibliographical notePublisher Copyright:
© 2016 Yang et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.