[Whatsapp Beta] Make it working with whatsapp-web.js#multidevice branch

[monossido]

Changes:

• Temporary git url in package.json in order to pull #multidevice branch of whatsapp-web.js
• "session.json" is deprecated and not needed in beta, I'have added a simple if to handle it. Not tested with standard no-beta whatsapp, but it should be ok.
• I've changed the QrCode generator lib, the old one kept throwing an error in the browser console "Error: code length overflow. (1716>1056)"
• In /getqr I've removed the check for session.json file in favour of "client.getState()"

Maybe more fixes are needed, I've tested only auhentication, send message and get Profile pic

Let me know if I should change something.

[monossido]

Keep in mind that with this PR in package.json there is a "beta" #multidevice branch from whatsapp-web.js, which is not stable. For example at the moment is crashing due to this https://github.com/pedroslopez/whatsapp-web.js/commit/0ee405cf1a3f1a80680f364e92eedddd94f88e87#r66636907

Maybe it would be better to create a new branch?