Add a weekly count of surveys shown, and prevent showing a new
notification if we've already reached that limit.

Simplifies and renames the setSurveyCount functions, and adds a new
alarm to reset the weekly count every seven days at midnight.
Merge branch 'master' into max-surveys-per-week

Conflicts:
	extension/background.js
Remove added milliseconds to daily survey count reset alarm. The
periodInMinutes handles this just as well when the first alarm is set to
the last midnight, and it looks less confusing.
Merge pull request #127 from christhompson/max-surveys-per-week

Set a weekly survey limit