Play Store link & analytics events.

This commit adds the Play Store button along with the App Store button to link
to the Android app. It fires Analytics events for clicks on each of the three
buttons.

I'm less than thrilled about the three different buttons on the page, none of them
the same size, etc. I'm thinking about creating a separate page for the apps (which
can include the full-size screenshot Lammert did a while back, as well as one for
the Android app), but moving the App/Play -Store buttons off the home page may
not be the brightest idea. Your thoughts are welcome.