Add Okra.Core.Portable PCL and link to possible portable code
Convert PCL classes to stubs
Replace references to NavigationMode (defined by Windows) to PageNavigationMode (defined by Okra)
Remove activation related interfaces from PCL (as this should be platform specific)
Rewrite IShareSourceManager and related infrastructure to be platform agnostic
Make adding data to ISharePackage genericly typed
Update IShareTargetManager and infrastructure to be platform agnostic
Add extension methods for strongly typed sharing
Remove IStorageManager from portable library (treat this as internal platform specific for now)