swc-windows-installer.sh: Rename ~/.nanorc to ~/nano.rc

The nano FAQ hints that this is the prefered path on MS Windows [1],
and Ethan White confirmed it in testing [2].

[1]: http://www.nano-editor.org/dist/v2.2/faq.html#3.9.1
[2]: https://github.com/swcarpentry/bc/pull/357#issuecomment-37482899
swc-windows-installer.sh: Use '~' in ~/nano.rc includes

On Wed, Mar 12, 2014 at 04:44:31PM -0700, Ethan White wrote [1]:
> Got it. We just need to change the `/c/Users/River/` in the include
> statements to `~/` and everything works great!

I also replaced an:

  if [ ... ]

condition with:

  if test ...

to match the rest of the script.

The underscore prefixes in the string-equality test are to avoid miss
interpretation in the pathological case that RCPATH starts with '!' or
some such.  It's hard to imagine how this could happen with the
current script, but... safety first ;).

[1]: https://github.com/swcarpentry/bc/pull/357#issuecomment-37483149
Merge branch 'master' into bc-namespaced

* master:
  swc-windows-installer.sh: Use '~' in ~/nano.rc includes
  swc-windows-installer.sh: Rename ~/.nanorc to ~/nano.rc