Updated underscore links to point to underscorejs.org

"http://documentcloud.github.io/underscore" is only updated through version 1.5.0, while "underscorejs.org" is current through 1.7.0. 

"underscorejs.org" is also the official homepage for the project, and is easier to remember if people want to reference again later. 

I've double checked both links and they behave exactly as the existing links do. The link pointing directly to #template still points directly to #template within the underscore documentation.