Add timestamp for tag-events

[hcfw007]

A few days ago Wenjin ask me if it's possible to add timestamp for tag and friendship events.
After reading corresponding code, I found that friendship payload contains timestamp already.
For tag events, @binsee told me all workplus events contains a timestamp. So here's what I'm gonna do:

Add a timestamp field for tag events in wechaty-puppet. juzibot/wechaty-puppet#25
Convert the timestamp to Date ojbect in wechaty. #28