Add support for heroku-agent [1]

This pull adds support for heroku-agent [1], which for the most part
involves a conditional override on the HTTP transport's `Dial` function if
the `HEROKU_AGENT_SOCK` env var hints that one might be running.

I've also included yet-unmerged changes to heroku-go which can be viewed
independently at heroku/heroku-go#12.

Please consider this feature somewhat experimental, and I'll prioritize its
removal if this project ends up faltering.

[1] https://github.com/brandur/heroku-agent