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.
Looks like this could be changed to a sync.RWMutex, and we can use RLock and RUnlock in the places where we're just reading from the data structure rather than mutating it. This will reduce some unnecessary blocking.