[SB][00000] process strikethrus only once per comment

* The private method comment_score doesn't need to care about blockers.
* Changed has_blocker? and comment_score to take string arguments
  instead of comment objects.
[SB][00000] only count most recent +/-1 per user

Addresses https://github.com/primedia/parliament/issues/6
[SB][00000] refactor #score
[SB][00000] #score should return 0 instead of nil

Was incorrectly returning nil when no +1 or -1 comments present.
Merge pull request #7 from primedia/sb_one_vote_per_user

SB one vote per user