Improve reverse proxy support

Should address edge cases and close #224 -- subject to field testing.

Does require the NGINX deployments use the following:

proxy_pass <http://documize-url:5001>;
proxy_set_header Host $http_host;
proxy_set_header X-Forwarded-For $remote_addr;

vendor/github.com/gorilla/handlers/handlers_go18.go

@@ -0,0 +1,21 @@
+// +build go1.8
+
+package handlers
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type loggingResponseWriter interface {
+	commonLoggingResponseWriter
+	http.Pusher
+}
+
+func (l *responseLogger) Push(target string, opts *http.PushOptions) error {
+	p, ok := l.w.(http.Pusher)
+	if !ok {
+		return fmt.Errorf("responseLogger does not implement http.Pusher")
+	}
+	return p.Push(target, opts)
+}