A humble pull request

Hi Aldo,

mitmproxy is an *amazing* tool; I've had a really fun dozen hours this weekend playing with it.

This patch lets you use transparent proxy mode on Mac OS X versions before Lion. (My hacking machine is an old Snow Leopard macbook).

I think this is strictly better, because it still works on post-Lion machines... but let me know if you'd like me to change it in any way. Thanks!