Use escapeshellarg() and DIRECTORY_SEPARATOR

Also:
- Fix unlink() error (On Windows, the file must be closed before
  unlinking)
Build devtool-win32.zip

devtool-win32.zip bundles a working execution environment containing:
- PHP 5.4.33 (from PHP.net)
- gettext 0.18.3 (from MinGW)
- devtool.phar (main devtool library)
- devtool.bat (stub script to setup working environment)