On OS X mavericks (10.9), port forwarding is not enabled in the system firewall, preventing the .dev domain resolution from working

Issue #415 mentions this, and has the fix of manually running `sudo sysctl net.inet.ip.fw.enable=1` which solves the problem, but only until the machine is rebooted.

Interestingly enough, Pow's `cx.pow.firewall.plist` launchd daemon runs this command:

`sh -c "ipfw add fwd 127.0.0.1,20559 tcp from any to me dst-port 80 in && sysctl -w net.inet.ip.forwarding=1"`

Unfortunately, launchd doesn't like this, because it doesn't know what `sh` is.

Changing `sh` to the absoute path of `/bin/sh` fixes this
Forgot to do it there, I'll push a commit in a couple of minutes that fixes that.