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.
Again, it would be lovely to do some checksum verification before trying to run the binary here. :)