Fix site search form to work with Google

Around beginning of November 2014 Octopress site search stopped working with Google, as you can see yourself by going to octopress.org and entering a search query.

The problem is apparently two 'q' inputs in the form. This commit fixes the problem by building q dynamically at submit time - the proper query and "site:yoursite.com" get concatenated in javascript to produce the desired effect.