add an api for setting custom DNS records

Works like this:

```curl -d "" --user email:password https://.../admin/dns/set/qname/rtype/value```

where the rtype and value default to "A" and the remote IP address of the request, so that a simple, empty POST to

```https://.../admin/dns/set/desktop.mydomain.com```

will point desktop.mydomain.com to the caller's IPv4 address.

closes #140

conf/nginx.conf

@@ -28,6 +28,7 @@ server {
 	rewrite ^/admin$ /admin/;
 	location /admin/ {
 		proxy_pass http://localhost:10222/;
+		proxy_set_header X-Forwarded-For $remote_addr;
 	}
 
 	# Roundcube Webmail configuration.