Add return self to service_interface

[webispy]

This patch add a 'return self;' to each functions in service_interface module to use method chaining technique. This helps developers write code more conveniently. Please refer to the example below.

var obj = service.createObject('/test');
obj.createInterface('test.iface')
   .addMethod('method1', ...)
   .addMethod('method2', ...)
   .update();

Signed-off-by: Inho Oh webispy@gmail.com

[Shouqun]

lgtm, thanks!