Improve auth data structure for upper.io

upper.io seems to expect a numeric io value.
Retrieve code from the temp server

Retrieve content from URL query string.
Detect error. Retrieve code if no error.

Also 1 minor improvement to logging in main.go
Add warning for integration testing in README.md

As the integration test now triggers default browser to complete the
OAuth2 process, a Linux GUI will proper default browser setting is now
a necessary.

Update README.md to state so.