Vendor all dependencies (Warning: Huge changeset.)

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

vendor/github.com/gorilla/websocket/examples/chat/main.go

@@ -0,0 +1,40 @@
+// Copyright 2013 The Gorilla WebSocket Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"flag"
+	"log"
+	"net/http"
+)
+
+var addr = flag.String("addr", ":8080", "http service address")
+
+func serveHome(w http.ResponseWriter, r *http.Request) {
+	log.Println(r.URL)
+	if r.URL.Path != "/" {
+		http.Error(w, "Not found", 404)
+		return
+	}
+	if r.Method != "GET" {
+		http.Error(w, "Method not allowed", 405)
+		return
+	}
+	http.ServeFile(w, r, "home.html")
+}
+
+func main() {
+	flag.Parse()
+	hub := newHub()
+	go hub.run()
+	http.HandleFunc("/", serveHome)
+	http.HandleFunc("/ws", func(w http.ResponseWriter, r *http.Request) {
+		serveWs(hub, w, r)
+	})
+	err := http.ListenAndServe(*addr, nil)
+	if err != nil {
+		log.Fatal("ListenAndServe: ", err)
+	}
+}