Add platform notification sample;

examples/push/example_platform.py

+import jpush as jpush
+from conf import app_key, master_secret
+_jpush = jpush.JPush(app_key, master_secret)
+
+push = _jpush.create_push()
+push.audience = jpush.all_
+push.notification = jpush.notification(alert="Hello, JPush!", ios=jpush.ios(alert="Hello, IOS JPush!", badge="+1", sound="a.caf", extras={'k1':'v1'}))
+push.platform = jpush.all_
+push.send()