Yosemite support: Replace deprecated ipfw with pf

These changes involve the solution proposed in #452.

Fixes #452, not sure what's going wrong with the tests.. On my local machine they all pass.

~~Final issue that now arises it that I can't see the Pow! index by visiting http://127.0.0.1:20559~~ This has been resolved by using localhost instead of 127.0.0.1

#### Installation

By using my slightly altered `install.sh` it's possible to install from a specified archive:
```sh
curl https://raw.githubusercontent.com/koenpunt/pow/ipfw-to-pf/install.sh | VERSION=0.4.4-pre ARCHIVE_URL=https://github.com/koenpunt/pow/releases/download/v0.4.4-pre/0.4.4-pre.tar.gz sh
```

Shortlink for the archive url: http://git.io/q4mxog