Adding endpoint for log retrieval on the minion

Adding logic for retrieving logs from docker containers.
Usage:
To returns snapshot of the logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6`  

 To tail 15 lines of logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6&tail=15`

To stream logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6&follow=1`

Related: 
https://github.com/GoogleCloudPlatform/kubernetes/issues/1235#issuecomment-55188947
Agree that returning json makes more sense, but dont see any way how to put the error into the responseWriter after that. Do you have any example or suggestion, which I am missing ? 
What I came with is to get rid of both lines and replace them with:
`http.Error(w, `{"message": "Missing containerID= query entry."}`, http.StatusBadRequest)`