middleware/proxy: healthchecks fixes (#183)

* middleware/proxy: add spray keyword

When spray is used, the proxy will, when all backend are down, spray to
each target. When not used, default to the old defaults: max 1 failure
and no spray. These defaults are also used when forwarding queries to
another CoreDNS instance.

Update the README with the new keyword.

* typos

* Make MaxFail = 1 again

* more reversals

middleware/etcd/stub_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/etcd/msg"
 	"github.com/miekg/coredns/middleware/test"
+
 	"github.com/miekg/dns"
 )