add basic chart for total successful checks overtime

This pull adds a basic '# success checks' time series, like this:
![screen shot 2014-08-20 at 2 02 25 pm](https://cloud.githubusercontent.com/assets/2533112/3988199/af0b6326-28ad-11e4-9ace-aea5ad1a79d6.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.