Add message revoke (littlecodersh#370)

* Link message revoke function to itchat

this is my first uploading code using github, apologize if it caused any inconvenience.

* Add message revoke function

function added at the end of file, after the position of send

* Update core.py

Add timeout for 'synccheck' and 'webwxsync'

to avoid:

1. get 1101 error frequently
2. won't logout even if network broken

Fix circular reference bug

Fix isAt, Self bugs in 1.3.0

Fix chatroom['Self'] bug

Fix missing emoji format [BR#297: youfou]

Merge 1.3.0 into master and update to 1.3.1

Fix image storage enhancement [ER#160: 6bigfire]

Issue littlecodersh#270 (littlecodersh#271)

Make login stoppable in qrCallback [BR#267: githubwing]