Add a pie chart.

The README said that there was a pie chart but I couldn't see it in the code. This patch adds basic functionality to push to a pie chart.

I did run into an issue when running the specs. I needed to change the User Agent in the `stub_gecko_post` helper because the UA is now being reported as `Faraday v0.8.7` not `Ruby`. I'm guessing it's a change between Faraday 1.8.7 and 1.8.1.