Detect if json object have new update

[jimyhuang]

We cache each word in json.
We can have a simple function to detect if the event have further progress by iterate the object between old and new json.

Everytime when update json, we should copy exists json to old one. So we have 2 cache at the same time.
one is event_name.json
the other is event_name.json.old

In the update progress, trigger mail event to send mail for subscriber.
May reference code here:
https://github.com/g0v/twangry/blob/master/mod/wikipedia.js#L233