fix bugs in PullRequest#score

Score should be zero when at least one [blocker] comment exists.
Ignore comment text inside ~~strike-thru markdown~~.
[blocker] pattern should be case-insensitive.
update README to describe comment parsing rules
fix handling of multiple strikesthrus

regex was greedy, so everything between first and last strikethru was
being removed

add a spec describing correct behavior
Merge branch 'master' into sb_scoring_bugfixes

Conflicts:
	lib/parliament/pull_request.rb
Merge pull request #4 from primedia/sb_scoring_bugfixes

SB scoring bugfixes