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.
Same here, we'll get less blocking using sync.RWMutex.