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.
Rather than calling decoder.Decoder() directly, you can just drop the pack on the DecoderRunner's InChan. You'll still need to check to make sure you *have* a decoder, of course, and if you don't you'll still call ir.Inject(), but you don't need to manage your own set of packs, and you don't need to worry about recycling or logging the error message, the DecoderRunner will handle all of that if necessary.