pylightning: use different decoration for init msg.

The next patch wants to decorate the methods with a compulsory
'usage' option, which doesn't make sense for init.  So I wanted
to change the init to its own decoration.

Made-to-work-by: @cdecker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/plugins/helloworld.py

@@ -19,7 +19,7 @@ def hello(plugin, name="world"):
     return s
 
 
-@plugin.method("init")
+@plugin.init()
 def init(options, configuration, plugin):
     plugin.log("Plugin helloworld.py initialized")