Add some docstrings to "public" methods.
Require otp_list to contain at least two OTPs.
Add test for verify_multi and different device ids.
Modify verify_multi so max_time_window is now in seconds.

Also modify it to throw if delta is smaller than zero.
Fix string interpolation.
Add some tests for verify_multi method.
Fix lint issue.
Use flake8 for lint step.
Use unittest2 under Python 2.6.