Add analytics tracking for Stats Insights 

Ref:  https://github.com/wordpress-mobile/WordPressCom-Analytics-iOS/pull/26

Events to add (in the iOS naming conventions):

```
WPAnalyticsStatStatsInsightsAccessed,
WPAnalyticsStatStatsPeriodDaysAccessed,
WPAnalyticsStatStatsPeriodWeeksAccessed,
WPAnalyticsStatStatsPeriodMonthsAccessed,
WPAnalyticsStatStatsPeriodYearsAccessed,
```
