Fix default prefix to include trailing slash.
Switch back to os.execute for running batch scripts.

Apparently io.popen has some problems on windows that os.execute does not.
The output given by io.popen is not particularly useful on windows, and I'm
pretty sure most users are using windows, so this is just a quick fix. At
some point I'll make it use io.popen on *nix to reintroduce the debug
output.