add google analytics (#128)

Author: mmcky

lectures/_config.yml

@@ -53,6 +53,7 @@ sphinx:
       og_logo_url: https://assets.quantecon.org/img/qe-og-logo.png
       description: This website presents a set of lectures on quantitative economic modeling, designed and written by Thomas J. Sargent and John Stachurski.
       keywords: Python, QuantEcon, Quantitative Economics, Economics, Sloan, Alfred P. Sloan Foundation, Tom J. Sargent, John Stachurski
+      google_analytics_id: UA-54984338-10
     mathjax_config:
       TeX:
         Macros: