Solves 404 Not Found when a new PHP version is released.

The script had a fixed file name ("php-5.4.17-nts-Win32-VC9-x86.zip") but when "php-5.4.18-nts-Win32-VC9-x86.zip" was released, the previous version was removed from the download directory ("http://windows.php.net/downloads/releases/").

To avoid having to change this name for every PHP release, the script will scrape the file name out of the HTML download page ("http://windows.php.net/download/").
Merge pull request #425 from fernandoacorreia/windows-install-php

Solves 404 on Windows when a new PHP version is released