Replace ipfw with pf.

See basecamp/pow#453

Fixes #26
Add fallback support for Mountain Lion and older.
Merge remote-tracking branch 'hoodiehq/master' into fix/yosemite-compatibility
We should be using our own anchor.
Use Travis multi-OS to build this on OS X
Try multi-OS build again, this time with nvm.
Try a ruby build, just for funs.
Merge pull request #31 from bobthecow/fix/yosemite-compatibility

Replace ipfw with pf.