refactor(config): Rename Hostname to Listen

Listen makes more sense for the coming socket-activation
feature.
chore(deps): Add go-systemd dep
bump(github.com/coreos/go-systemd): 700560af03f8e1df35839041745c9f1fccba7c72
refactor(*): break things apart
feat(health): Add basic health handler
feat(fd): socket-activation!
Merge pull request #1 from bcwaldon/socket-activation

Hack together some socket-activated goodness