New input plugin for Docker containers: DockerLogInput

Solves: #1092
This is a follow-up on PR #1095

This PR implements DockerLogInput, an input plugin based on Logspout for sending logs from Docker containers into the heka pipeline.

@rafrombrc Something like this? I renamed the plugin to DockerLogInput and wrote some basic docs for it.
`go-dockerclient` is a transitive dependency, not a Heka plugin, so you can use the `git_clone` command here instead of `add_external_plugin`.