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.
Here's another direct use of `log.Fatal`, no bueno. I haven't dug in far enough to know what it means if the `events` channel is closed, but, again, we should be delegating back to Heka when we have failure cases, not unilaterally shutting everything down.