[WIP] update mac configure script and add messaging about common port forwarding issue

update mac configure script and add messaging about port forwarding until we maybe use a better solution like how libtorrent does this, or RPC & UDP like this https://github.com/bmuller/rpcudp to avoid most port forwarding issues.
If `2.7.8` is the version that gets installed by `brew`, that's perfectly fine by me. Maybe I should have clarified that I am using `2.7.6` on Linux Mint 17, not on Mac OS X (I don't have a Mac). :)

I don't think there are significant differences between 2.7.6 and 2.7.8. Just install the default one.