Correct instances of '+ +' and '+ -' to protect from minification errors...

I'm working on an app that will include d3 for a data visualization piece. Our process of minifying d3.js into our app was causing several 'Invalid left-hand side expression in postfix operation' errors caused by sequences of '+ +' and '+ -'. In order to avoid this issue, wrapping the modified variables in parenthesis prevents '+ ++' from becoming '+++' and '+ -' from becoming '+-' in the minified code and resolves the issue.

There is no functional change to the library in this update; it's simply a slight syntax change.