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 have at times gotten intermittent failures building w/ `make -j8` on my linux box, I think b/c some steps start before a dependent step has finished. I'm wary of turning this on here.