Add a check to showSurveyNotification to only proceed if we're under
some maximum number of surveys to show each day.

Add an alarm to reset the count of surveys shown to 0 each day.
Add missing MAX_SURVEYS_PER_DAY constant
Add handler for the daily survey count reset alarm.
Merge branch 'master' into maxsurveys

Conflicts:
	extension/background.js
Reset survey count at midnight each day
Merge pull request #42 from christhompson/maxsurveys

Throttle surveys shown per day