Make CoreDNS a server type plugin for Caddy (#220)

* Make CoreDNS a server type plugin for Caddy

Remove code we don't need and port all middleware over. Fix all tests
and rework the documentation.

Also make `go generate` build a caddy binary which we then copy into
our directory. This means `go build`-builds remain working as-is.

And new etc instances in each etcd test for better isolation.
Fix more tests and rework test.Server with the newer support Caddy offers.

Fix Makefile to support new mode of operation.

core/dnsserver/address.go

@@ -0,0 +1,44 @@
+package dnsserver
+
+import (
+	"fmt"
+	"net"
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+type zoneAddr struct {
+	Zone string
+	Port string
+}
+
+// String return z.Zone + ":" + z.Port as a string.
+func (z zoneAddr) String() string { return z.Zone + ":" + z.Port }
+
+// normalizeZone parses an zone string into a structured format with separate
+// host, and port portions, as well as the original input string.
+func normalizeZone(str string) (zoneAddr, error) {
+	var err error
+
+	// separate host and port
+	host, port, err := net.SplitHostPort(str)
+	if err != nil {
+		host, port, err = net.SplitHostPort(str + ":")
+		// no error check here; return err at end of function
+	}
+
+	if len(host) > 255 {
+		return zoneAddr{}, fmt.Errorf("specified zone is too long: %d > 255", len(host))
+	}
+	_, d := dns.IsDomainName(host)
+	if !d {
+		return zoneAddr{}, fmt.Errorf("zone is not a valid domain name: %s", host)
+	}
+
+	if port == "" {
+		port = "53"
+	}
+
+	return zoneAddr{Zone: strings.ToLower(dns.Fqdn(host)), Port: port}, err
+}