Fix killing in Windows

Hi, all

In windows, the second time of build always fails, saying "cannot create gin-bin.exe: Permission denied." This is because Go does not support Interrupt signal in Windows.

Though I fixed this, there is still a problem that errors of killing are not showed because of https://github.com/codegangsta/gin/blob/96e94cfee0ab2dc348501d64c9c28eb2ccde1605/main.go#L93 .

Thank you