WhatsApp uses the XMPP protocol, below you can find a link to the open repository for 2 WhatsApp web interfaces using PHP or Python your choice.
https://github.com/venomous0x/WhatsAPI
I think you can write an ajaxified version with the above as its backend code, of course you need to add all the functionality to it and communication with the client side, but it's definitely doable in a couple of days.
Edit
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The reason why the above works is because of whatsapp's very unsecure authentication method. Indeed it uses IEMI as password
http://samgranger.com/whatsapp-is-using-imei-numbers-as-passwords/
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Fork the repo and if you can come up with something interesting, please share :)