Adding skeleton for WPStats.
Added skeleton implementation of WPStatsMixpanelClient.

- Created WPStatsClient protocol
- Routed calls to WPStats -> WPStatsMixpanelClient
Added a method to allow tracking for properties.
Adding class to track map instructions from generic stat to mixpanel.

Created WPStatsMixpanelClientInstructionsForStat to convert a generic
WPStat enum to a set of different events, super properties, etc. that
we want to send up to Mixpanel.
Adding stat tracking for themes.
Added stats for various reader actions.

- Reader - Liked Article
- Reader - Commented on Article
- Reader - Accessed Reader
- Reader - Liked Article
- Reader - Infinite Scroll
- Reader - Opened Article
- Reader - Loaded Freshly Pressed
- Reader - Reblogged Article
Adding tracking for stats.
Adding tracking for a editor events.
Added stats for notifications.
Adding a missing stat (Application Opened).
Converted and removed WPMobileStats top level items.

- StatsPropertySidebarClickedReader
- StatsPropertySidebarClickedNotifications
- StatsPropertySidebarSiteClickedPosts -> WPStatOpenedPages
- StatsPropertySidebarSiteClickedPages -> WPStatOpenedPosts
- StatsPropertySidebarSiteClickedComments -> WPStatOpenedComments
- StatsPropertySidebarSiteClickedStats
- StatsPropertySidebarSiteClickedViewSite -> WPStatOpenedViewSite
- StatsPropertySidebarSiteClickedViewAdmin -> WPStatOpenedViewAdmin
- StatsPropertySidebarSiteClickedThemes
- StatsPropertySidebarSiteClickedMediaLibrary -> WPStatOpenedMediaLibrary
- StatsPropertySidebarClickedSettings -> WPStatOpenedSettings
- StatsPropertySidebarClickedQuickPhoto
Removed WPMobileStats reader related calls.

- StatsEventReaderOpened
- StatsEventReaderHomePageRefresh
- StatsEventReaderInfiniteScroll;
- StatsEventReaderSelectedFreshlyPressedTopic;
- StatsEventReaderSelectedCategory;
- StatsEventReaderOpenedArticleDetails;
- StatsEventReaderPublishedComment;
- StatsEventReaderReblogged;
- StatsEventReaderLikedPost;
- StatsEventReaderUnlikedPost;
- StatsPropertyReaderOpenedFromExternalURL;
- StatsPropertyReaderOpenedFromExternalURLCount;
- StatsEventReaderOpenedFromExternalSource;
Adding superPropertyToFlag property to mixpanel instructions class.
Removing WPMobileStats calls for reader detail and web view sharing.
Removed WPMobileStats calls for notifications.
Removing WPMobileStats post/page related calls.
Removing WPMobileStats calls for comments and settings.
Removed WPMobileStats calls for notifications, nux.
Removed WPMobileStats calls to track general super properties.
Removing WPMobileStats calls to track reader super properties.