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.
Hrm... not too stoked on this library's use of its own `assert` function, which delegates directly to `log.Fatal`. A dependency library ideally wouldn't even be directly writing to stderr or stdout (since the app may have its own way to managing log output), much less killing the entire app. Ideally this function would instead return an error, which allows Heka itself to decide what to do about the failure.