Support Circle CI

[original circle_ci.json is here](https://circleci.com/docs/api#build),
I modified a little to get closer to a real request.

preview hook:
![_2014-06-20_00_55_48](https://cloud.githubusercontent.com/assets/167012/3329818/9689e926-f7ca-11e3-9256-24530b44b105.png)

Would you like to use Gravatar? It looks beautiful on retina display.

``` ruby
icon_url gravatar('c37e5f37d63fe731f139b7da3f759139')
```