This article was republished on March 21, 2014, to correct errors introduced into the equations during typesetting. Please download this article again to view the correct version. A formal notice of republication will be published shortly.

In the article PDF, in the Materials and Methods section, there were errors in Equations (6), (9) and (10), and in the equations in the text below Equation (6), above Equation (10), and in the paragraph below Equation (10). In all these cases, T should be superscript for "matrix transpose".