Dockerized the testrunner so that buildpacks can run tests
within a container. A build pack projects would need to create
a Dockerfile similar to this:

  FROM heroku/testrunner
  ADD . /app/buildpack

And then run a command like this:

  docker build --rm -t heroku/buildpack-scala .
  docker run -i heroku/buildpack-scala /app/testrunner/bin/run /app/buildpack
reworked the dockerfile so that buildpacks don't need a dockerfile
added docker instructions to the readme
cleaned up the shunit download and unpackaging
moved entrypoint args into a cmd
Merge pull request #5 from jkutner/dockerized

Dockerized