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/doc.go

@@ -0,0 +1,9 @@
+/*
+Package handlers is a collection of handlers (aka "HTTP middleware") for use
+with Go's net/http package (or any framework supporting http.Handler).
+
+The package includes handlers for logging in standardised formats, compressing
+HTTP responses, validating content types and other useful tools for manipulating
+requests and responses.
+*/
+package handlers