finally adding shell lessons and cheatsheets for shell and sql

Conflicts:
	lessons/shell/00-intro.md
	lessons/shell/README.txt
Cleaning up shell lesson.

This is to bring the directory in sync with datacarpentry/datacarpentry@9aa13270cf7ab0dc47b9b7f6d280f5305465a17b.
Also, removing data files as they are redundant with those under the data/
directory.