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.

middleware/bind/README.md

@@ -0,0 +1,21 @@
+# bind
+
+bind overrides the host to which the server should bind. Normally, the listener binds to the
+wildcard host. However, you may force the listener to bind to another IP instead. This
+directive accepts only an address, not a port. 
+
+## Syntax
+
+~~~ txt
+bind address
+~~~
+
+address is the IP address to bind to.
+
+## Examples
+
+To make your socket accessible only to that machine, bind to IP 127.0.0.1 (localhost):
+
+~~~ txt
+bind 127.0.0.1
+~~~