Add etcd middleware

This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.

A Corefile like:
.:1053 {
    etcd miek.nl
    proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.

The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free

middleware/etcd/path.go

@@ -0,0 +1,46 @@
+package etcd
+
+import (
+	"path"
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+// Path converts a domainname to an etcd path. If s looks like service.staging.skydns.local.,
+// the resulting key will be /skydns/local/skydns/staging/service .
+func (e Etcd) Path(s string) string {
+	l := dns.SplitDomainName(s)
+	for i, j := 0, len(l)-1; i < j; i, j = i+1, j-1 {
+		l[i], l[j] = l[j], l[i]
+	}
+	return path.Join(append([]string{"/" + e.PathPrefix + "/"}, l...)...)
+}
+
+// Domain is the opposite of Path.
+func (e Etcd) Domain(s string) string {
+	l := strings.Split(s, "/")
+	// start with 1, to strip /skydns
+	for i, j := 1, len(l)-1; i < j; i, j = i+1, j-1 {
+		l[i], l[j] = l[j], l[i]
+	}
+	return dns.Fqdn(strings.Join(l[1:len(l)-1], "."))
+}
+
+// As Path, but if a name contains wildcards (* or any), the name will be
+// chopped of before the (first) wildcard, and we do a highler evel search and
+// later find the matching names.  So service.*.skydns.local, will look for all
+// services under skydns.local and will later check for names that match
+// service.*.skydns.local.  If a wildcard is found the returned bool is true.
+func (e Etcd) PathWithWildcard(s string) (string, bool) {
+	l := dns.SplitDomainName(s)
+	for i, j := 0, len(l)-1; i < j; i, j = i+1, j-1 {
+		l[i], l[j] = l[j], l[i]
+	}
+	for i, k := range l {
+		if k == "*" || k == "any" {
+			return path.Join(append([]string{"/" + e.PathPrefix + "/"}, l[:i]...)...), true
+		}
+	}
+	return path.Join(append([]string{"/" + e.PathPrefix + "/"}, l...)...), false
+}