AcknowledgmentsThis work was financ

Acknowledgments
This work was financed by grants from the project of The National
Natural Science Funds (51308536), the Specialized Research
Fund for the Doctoral Program of Higher Education
(20130008120018), and Fund for supervisor of excellent PhD
dissertation of Beijing City (20131001903). We greatly appreciate
the critical and constructive comments from the anonymous reviewers,
which helped improve this manuscript.