Add IP to public machine struct

[ezekg]

Supersedes #15. Adds the IP property to the public Machine struct, but does not implement functionality to set a machine's IP attribute during license.Activate(). This allows the IP property to be accessible via a machine file.

One possibility for setting the IP could be:

func (l *License) Activate(fingerprint string, components ...Component) (*Machine, error) {
	client := NewClient()
	hostname, _ := os.Hostname()
	conn, _ := net.Dial("udp", "keygen.sh:80") // this doesn't actually hit the network
	defer conn.Close()

	params := &Machine{
		Fingerprint: fingerprint,
		Hostname:    hostname,
		Platform:    runtime.GOOS + "/" + runtime.GOARCH,
		IP:          conn.LocalAddr().String(),
		Cores:       runtime.NumCPU(),
		LicenseID:   l.ID,
		components:  components,
	}

	machine := &Machine{}
	if _, err := client.Post("machines", params, machine); err != nil {
		return nil, err
	}

	return machine, nil
}

But that risks not using the right IP, so it may be best to leave this to the end user somehow.