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.
If err is not nil, then hostname is *already* the empty string. You can either use `hostname, _ := os.Hostname()` or (even better) explicitly set the hostname to "unknown" (or some other sentinel value) when there's an error.