Add a Nano installer for Windows

Based on discussions here:
http://lists.software-carpentry.org/pipermail/tutors_lists.software-carpentry.org/2013-September/000934.html

It seems like it would be useful to have a Nano installer for Windows users. This is my attempt. It does the following:

1. Downloads and installs Nano into the user's home directory
2. Adds Nano to the path
3. Makes Nano the default editor

This has been tested on a single Windows 7 machine.