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/config.go

@@ -0,0 +1,38 @@
+package dnsserver
+
+import "github.com/mholt/caddy"
+
+// Config configuration for a single server.
+type Config struct {
+	// The zone of the site.
+	Zone string
+
+	// The hostname to bind listener to, defaults to the wildcard address
+	ListenHost string
+
+	// The port to listen on.
+	Port string
+
+	// The directory from which to parse db files, and store keys.
+	Root string
+
+	// Middleware stack.
+	Middleware []Middleware
+
+	// Compiled middleware stack.
+	middlewareChain Handler
+}
+
+// GetConfig gets the Config that corresponds to c.
+// If none exist nil is returned.
+func GetConfig(c *caddy.Controller) *Config {
+	ctx := c.Context().(*dnsContext)
+	if cfg, ok := ctx.keysToConfigs[c.Key]; ok {
+		return cfg
+	}
+	// we should only get here during tests because directive
+	// actions typically skip the server blocks where we make
+	// the configs.
+	ctx.saveConfig(c.Key, &Config{Root: Root})
+	return GetConfig(c)
+}