Initial commit of solution to `ls'

* code/ls-orig.scm: original code from Jez
* code/ls.scm: copy for hacking
replace string-starts-with? with string-prefix?

Removes a custom function

* code/ls.scm (string-starts-with?): removed
Week #5 Challenge, #2 Solution

* ch01_bin.texi: new file, split from g100
* ch02_web1.texi: new file, split from g100
* code/ls-orig.scm: update with Jez Ng's latest version
* code/ls.scm: updated
* g100.texi: include chapter files
* img/build.sh: script to extract images from alphabet picture
* img/stencil.gif: alphabet picture
* index.html: update with new solutions, theme, and challenges