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.
It's generally considered better practice to use the attribute name spelling:

```go
    return &DockerLogInputConfig{
        Endpoint: "unix:///var/run/docker.sock",
    }
```