Add '# successful checks' by source time series

This PR adds a chart with '# successful' series for each source, like this:
![screen shot 2014-08-20 at 8 19 47 pm](https://cloud.githubusercontent.com/assets/2533112/3991445/754c8dce-28e2-11e4-9524-6367e406911b.png)

I'm generating these charts with plot.ly, so you'll need to add two more environment variables (I'll DM you my credentials for now):

PLOTLY_USERNAME
PLOTLY_API_KEY
In the future we should separate out the public component so these charts can be embedded in the readme.