Add PuTTY instructions to the SSH guide

This pull request is a modification to the existing _SSH into your VM_ guide that adds a new set of _Tutorial Steps_ for Windows/PuTTY users. There are screenshots included, but no videos.

(You can preview the changes on [my Koding VM](http://gemma.kd.io:8080/docs/guides/ssh-into-your-vm/#tutorial-steps-for-putty-on-windows) until it goes down.)