2 bug fixes, README update

Oops, committed 2 other changes to master too. Updating this pull request. :)
- If the script is run from the command line not using the database, send_email will throw an exception because the dlog call uses "email" instead of config['EMAIL_TO'].  This patch just makes it the dlog use config['EMAIL_TO'] instead.
- Applied SarahPythonista's patch and tested backwards-compatibility on a 1-leg roundtrip flight. Don't have a multi-leg flight to test on.
- Updated the README adding a KNOWN ISSUES section.  The section explains the unpredictable database behavior (which I've experienced, and is in issue#10: https://github.com/aortbals/southwest-checkin/issues/10) and provides a tested workaround.