Add Result function to query Build Results endpoint

This PR adds support for the [Build Result Platform API endpoint][1].


[1]: https://devcenter.heroku.com/articles/platform-api-reference#build-result
the intent was to get things under the heroku banner. I change it in Endosome in [this pr](https://github.com/heroku/endosome/pull/71/files#diff-8f4e7c5dd8ff2f5d56b74e1938c6382aL11)

I can revert it if I'm changing too many things at once. Honestly, there was a heroku naaman source and a heroku source; seemed like the heroku one was the one to use. I think there was an email about this kind of thing this (or last) week.

I'm indifferent here really; in hindsight this change is unrelated and shouldn't have been included.