Fix table display on GitHub

Convert tables into a format that GitHub will render properly.

This commit introduces an ugly workaround to keep PDF generation using Pandoc functional: A dependency on `sed` for building the student handout. See the commit message on https://github.com/ccharles/PHP-Course/commit/5d1fe670ef41dfabcbf753ed6fb7d90eda9f0274 for details.