Track the time of the last survey notification, and only show another
one if MINIMUM_SURVEY_DELAY has elapsed.
Merge branch 'master' into limit-repeat-surveys
Merge pull request #110 from christhompson/limit-repeat-surveys

Track the time of the last survey notification, and only show another one if minimum delay has elapsed
Merge branch 'master' into max-surveys-per-week

Conflicts:
	extension/background.js