Change working directory of PHP script execution context

This commit sets the working directory of PHP script execution context
to the directory containing the PHP script, instead of the directory
of the Middleman project, ensuring that relative paths within the PHP
script will resolve to the same absolute paths as if the PHP script
had been run from a typical web server.
Added injection of PHP $_REQUEST variable

Setting $_REQUEST equal to the merger of $_ENV, $_GET, $_POST,
$_COOKIE, and $_SERVER, per the default value "EGPCS" of the
variable_orders directive in php.ini.  For more information,
see http://php.net/manual/en/ini.core.php#ini.variables-order
Using the default value "EG php.ini variable_order
Added Middleman config variable to set middleman-php debug mode