Get update filename from Content-Disposition header if available

Hi! Thanks a lot for winsparkle, we've been using it since version 0.2 without a glitch!

On to the code!

Our appcast URL is something like http://domain/foo.php?update which then returns the filename in the Content-Disposition header. This pull request uses the content of that header if available as the temporary filename for the download.

PS: My C++ kungfu is not very strong, if you'd like to see this done differently please let me know and I'll try to make it right!