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/safe_t/manifest.json

@@ -1,7 +1,8 @@
 {
+  "name": "safe_t",
   "fullname": "Safe-T mini Wallet",
   "description": "Provides support for Safe-T mini hardware wallet",
   "requires": [["safetlib", "github.com/archos-safe-t/python-safet"]],
   "registers_keystore": ["hardware", "safe_t", "Safe-T mini wallet"],
   "available_for": ["qt", "cmdline"]
-}
+}