SB one vote per user

This PR changes the scoring such that only the most recent +1 or -1 from each user is taken into account.
This address issue https://github.com/primedia/parliament/issues/6.

Btw: In the process I introduced a new private method scores_by_username which is a hash with each +1 or -1 score by username.  This will come in handy later when we add the option of requiring +1 from a specific set of usernames (the "required" config option described in the README).