#59 Add device code authenticator

[Dick Marinus]

Device code authentication is a bit different from call back authentication as for device code authentication a (short) code has to be copied from davmail to the web browser.

The advantages of this are a slightly better user experience and some appid's don't allow/support call back authentication.

I think davmail could be refactored a bit to make these changes a bit more clean (make the call back url optional, make a resource a single definition, move json parsing)

Please let me know what you think of this.