Remove generated files under wiki/ as part of cleanup
Make the wiki/ directory part of the repo

This directory is required for the site to work correctly. Rather
than rely on it being manually setup, ensure that it is always
included in a checkout and just ignore any generated *.html files
inside it.
No need to check for wiki/ directory

As the wiki/ directory is now in the repo by default, there's
no need to check for it and/or create it in the gfm.php script.
Set execute bit on regen.php
Small readability and consistency fixes for README
Clean up indentation and remove commented-out code
Add a sample Apache conf file and instructions

On XAMPP, the provided .htaccess file doesn't work out of the box,
and resists being used if the site is served from a subdirectory
(http://localhost/crosswalk-website for example).

I ended up using a separate configuration file on the directory
containing the project, and ignoring the .htaccess file, replacing
its rewrite rules with my own. This doesn't impact on the use
of .htaccess in the production version of the site, but makes
setup easier in environments where .htaccess isn't suitable.
More tweaks to setup instructions in README
Remove my too-specific setup instructions
Add simplified HACKING instructions
Use one script to do all CSS processing

Moved all the CSS processing code into one location,
generate.css.php.

This meant that using __FILE__ to determine the basename for
the output file was no longer valid. Instead, use
$_SERVER['SCRIPT_URI'], which refers to the original script, rather
than any scripts being require'd.

The main impetus for this was so I could set the sass path
explicitly for the script, rather than just calling "sass" (there
may be situations where the Apache user doesn't have sass on its
path). "sass" is now the default, but can be overridden in Apache
config with "SetEnv SASS /path/to/sass". The HACKING.md file
explains how.