Nature Communications 3: Article number: 868 (2012); Published: 29 May 2012; Updated: 26 February 2013.

This article contains typographical errors in equations (3) and (4) that were introduced during the production process. These errors do not affect the analysis or results presented in the paper. Equations (3) and (4) should read as follows.

In addition, the mathematical term in the sentence immediately following equation (5) also contains a typographical error, and should read: .