Add Google Analytics tracking for user actions.

- Dashboard Page: How many users on dashboard page
- Dashboard Page: Tweet Button on graphs to find out which graph is
tweeted
- Dashboard Page: Refresh button on Github graph
- Comparison Page: How many people initiated compare request and how many
actually completed the flow
- Community Page: How many people on community page
- Signup Page: How many people on signup page
- Signup Page: How many people are actually signing up (click on Signup with Github button)