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
I'm not suggesting a specific implementation yet.  I'm just asking what you expect to happen when:
- user 1 tries to GetKubeletContainerLogs(...) with opts.Follow = true
- user 2 tries to do something else.