fixed: autoupgrade fails on windows if autoupgrade_dir has been modified

If ``autoupgrade_dir`` is modified on a windows client, burp fetches files (``package.exe`` and ``script.bat``) from the server to the correct path, but the installation batch file fails because it references a different directory than the one it is in. I fixed this using [batch parameter modifiers](http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/percent.mspx?mfr=true) in ``script.bat``.

Please review and pull.