Added support for Gmail OAUTH2 authentication to increase security. This means no password is needed any more and also no need to enable the option "allow less secure apps to access your account". Please note that you can't convert already set-up accounts to use the new authentication mechanism, you need to add a new account: https://youtu.be/7ieY5uK4YEE
Modifications to comply with the new permission model of Android 6.0
The app uses now material-design address chips when composing messages