Listen for new releases and restart when they appear

Listen for new releases by periodically polling the Heroku API. When a
new release appears, call into the previously unused `restarter`
function which SIGTERM's the child process group and gives it ten
seconds to stop. A SIGKILL is then sent if the process has still not
exited.
Merge pull request #3 from fdr/brandur-listen-for-releases

Listen for new releases and restart when they appear