StatsViewController implements WPStatsViewController from pod

Closes #1790 

Switches over to use WordPressCom-Stats-iOS pod for implementing stats view.  Removed service from project and made StatsViewController extend WPStatsViewController as a consumer of the project might do.  Still a bit rough around the edges but wanted to get this into WPiOS for review and testing.
'author' or 'authors' is required by ````pod spec lint````