shell: Rename the new-style lessons from 'bash' to 'shell'

This is mostly about utilities with a few references to the basic
POSIX shell, and hardly at all about Bash itself (as distinct from
POSIX shells more generally).  'shell' is also consistent with the
existing:

* _includes/guide-shell
* lessons/ref/shell.html
* lessons/swc-shell
* lessons/thw-shell

If we weren't looking for a short-and-sweet directory name, I'd prefer
"Shell & Utilities" which the POSIX folks presumably put some thought
into [1] ;).  These lessons are mainly about generic POSIX concepts
(filesystem paths [2], basic utilities [3]).  However, I think calling
the directory 'posix' would confuse people :p.  'shell' is both
familiar, roughly accurate (because we use the shell to teach the
lessons), and not over-specific.  If you want to talk about
associative arrays (/shell/hash/intermediate? ;), than we're talking
Bash instead of POSIX ;).

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/xcu.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html
Preparing material for Version 5 novice release
Corrects runtime calculation for goostat and goodiff. 300 choose 2 is 300*299/2. Also the conversion from seconds to weeks was off.
Minor typo
Corrects comments in middle.sh script
Clarifies what we mean by a few extra key strokes
Edits on top of @BernhardKonrad's corrections
add cat to be consistent with previous lessons
Merge branch 'v5alpha' of https://github.com/gvwilson/bc into v5alpha
Specify key points
Slight edit to @BernhardKonrad's chnages
1.  Making filenames in `find` example match filenames in diagram.
2.  Using `-print` with `find`.
Switching to explicit <pre> blocks with styles instead of fences
Fixing formatting of pipe-and-filter challenges
Removing explicit 'level' keys from Markdown files
*   Getting rid of old cheat sheets
*   Indenting error output cells
*   Removing explicit 'level' keys from Markdown files
*   Storing the generated files so that people who don't use those tools won't have to regenerate them.
*   Modifying .gitignore to reflect this.
*   Updating the Makefile to run Jekyll exactly once.
*   Command to install on the server
*   No longer worrying about making a page of images
*   Better (more guessable) name for the target that builds the website
*   Fixing image paths
*   Fixing up glossary entries
Merge branch 'sticking-with-jekyll' of github.com:gvwilson/bc into sticking-with-jekyll
Make Challenge 2 in the novice shell lesson 5 consistent

This challenge used two different file names, neither of which was
consistent with part of the problem description. I chose the
longest.sh file name and made the challenge consistent for this
choice.
Typo fix in Challenge 4 of Novice Shell 5
Converting shell lesson Markdown files to use div + fence block