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.
I'd really like to see a SHA1 checksum check here to make sure the binary is right before we try to use it.