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