Check for existing token during two-factor login

OAuth2 token notes must be unique so without this check a
login from a 2nd device will fail when it tries to create an
authorization and one already exists.
Prepare 1.9 release
Update README.md
Update README.md
Merge pull request #555 from rodrigoargumedo/master

README.md correction
Update README.md
Merge pull request #556 from MaximLoguncov/master

Update README.md