Make it possible to create zip plugins from internal plugins

specifically:
 - add 'name' field to manifest.json
 - make 'version' optional in contrib/make_plugin
 - fix import in jade plugin

electrum/plugins/swapserver/manifest.json

@@ -1,4 +1,5 @@
 {
+  "name": "swapserver",
   "fullname": "SwapServer",
   "description": "Submarine swap server for an Electrum daemon.\n\nExample setup:\n\n  electrum -o setconfig enable_plugin_swapserver True\n  electrum -o setconfig swapserver_port 5455\n  electrum daemon -v\n\n",
   "available_for": ["cmdline"]