Fix for #33: Closed socket.

#33 seems to be related to isaacs/npm-www#74, whose root cause is in turn covered by joyent/node#2318.

The problem is that child_process.spawn ignores the PATHEXT completion for batch files. Thus, just invoking `sass` or `compass` is not enough, since it should be `sass.bat` or `compass.bat`. The proposed workaround is to call `cmd.exe` as an intermediary shell that takes care of that resolution. I took the fix from isaacs/npm-www#74. 

Licenses should be compatible (MIT and BSD).