Add signer backend

This commit adds a simple go backend calling openssl ca to sign CRS
coming from the client. The JavaScript code in src/index.html has been
extended to send requests to the sign endpoint and display the resulting
certificate in a separate div element.

A script setup_example_ca.sh and an openssl configuration file ca.cnf
has been added to allow quick setup of a simple example CA.

setup_example_ca.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+if [ ! -d "exampleca" ]; then
+  mkdir -p exampleca/newcerts
+  touch exampleca/index.txt
+  umask 077
+  mkdir exampleca/private
+  openssl req -new -x509 -keyout exampleca/private/ca.key.pem -out exampleca/ca.crt.pem -days 3650 \
+    -subj "/CN=Example CA" -nodes -newkey rsa:3072 -addext "basicConstraints=critical,CA:true,pathlen:0"
+  chmod +r exampleca/ca.crt.pem
+fi