Add Dockerfile(s) to Heka repo.

This PR contains a few Dockerfiles, as well a script for building
a lightweight Docker image with Heka installed via a deb package, without
all of the source build artifacts.

There is also a fully working example/demo in the docker/example directory
which demonstrates how you could use docker with Heka.
Localhost works, but it brings you to the dashboard which asks you where you want to go (blank, guided, ect). This takes you directly to the guided dashboard which should show actual data. Although at this point there won't be any. I prefer being consistent when I refer to the URL we're going to since we'll be going to this one later in the example once we've finished.