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/").
Install mercurial on server (resolves #417)
Check installation errors

In case of error the command and status code will be shown.
If --install-error-action=abort the installation will be aborted.
Proper flushing of install logs
Merge branch 'windows-install-php' into facorreia/azure
Merge branch 'install-mercurial' into facorreia/azure
Merge branch 'install-error-checking' into facorreia/azure