Replace Python with Shell

In the box "Text vs. Whatever" it say Python when should say
Shell.

I move this box two paragraphs up to be closer of the lesson
part about writing the file 'middle.sh'.
Replace "Shell" by "shell"

As point out by @ahmadia we don't use "shell" capitalized (see the
Wikipedia entry: http://en.wikipedia.org/wiki/Shell_(computing)).
Cite shell parameter expansion

This is part of Issue #54.

In this commit we mention the shell parameter expansion to make
easy to students go deep if they need.

We also add a note about use ${some-var} instead of $some-var.
Fix some typos

Thanks to @ahmadia and @ethanwhite for the review.
Converted exercises and tutorial for thw-testing to ipython notebook.
made some minor updates
Added @wltrimbl's suggested fixes from #28

* Removed Unicode debris
* Added exception link
Merge pull request #47 from r-gaia-cs/text-processor

Replace Python with Shell
Merge pull request #59 from r-gaia-cs/teach06-expansion

Cite shell parameter expansion