Add a -signal option to docker kill command

This is to allow us to send custom signals to docker containers. This will help in the following cases.

* Send SIGHUP after logrotate (if the logs are being handled external to the container)
* manage the container with tools like Supervisord / upstart (INT, QUIT to stop the container)

NOTE: This is not tested yet. Will update the pull request after testing it thoroughly (sometime over the weekend). Do share any comments/thoughts.