Log API requests in Google Analytics

![tba-ga](https://f.cloud.github.com/assets/2099988/1046408/079f7134-103c-11e3-975f-e0c8a0208c43.png)

Adds new Sitevar `google_analytics.id` which looks like:

    {"GOOGLE_ANALYTICS_ID": "UA-XXXXXXXX-Y"}

Closes #185