add app.json

I still don't have this thing running locally, but the `app.json` manifest is working!

Eventually support for this will make its way into the Heroku CLI and will also be honored on first push with a regular old `git push heroku master`, but for now you have to use the nascent Heroku Build API. Here's the current rough-around-the-edges way to spawn a new app with a bash function:

```sh
$ concoct zeke/roots-buildpack-sample
```

```sh
concoct() {
  host="https://nyata.herokuapp.com"
  tarball_url="https://codeload.github.com/${1}/legacy.tar.gz/master"
  token=$(heroku auth:token)

  echo "\nConcocting..."
  echo $tarball_url

  api_response=$(curl \
    -u :$token \
    -X POST \
    --silent \
    -H 'Content-Type: application/json' \
    -d "{ \"source_blob\": { \"url\": \"$tarball_url\" } }" \
    $host/builds)

  echo "\nAPI Response"
  echo $api_response

  echo "\nBuild ID"
  build_id=$(echo $api_response | jq -r .id)
  echo $build_id

  echo "\nBuild Status URL"
  status_url="$host/builds/$build_id"
  echo $status_url

  # status is a reserved word in bash
  state="pending"
  while [ $state == "pending" ]; do
    status_response=$(curl \
      -u :$token \
      --silent \
      -H 'Content-Type: application/json' \
      $status_url)
    state=$(echo $status_response | jq -r .status)
    echo "\nStatus"
    echo $status_response
    sleep 1
  done
}
```