Create a Portable Class Library

Since Caliburn.Micro is moving to PCL with v2.0, moving to PCL will simplify maintenance.

Here are the anticipated steps:
- [x] Updgrade to Caliburn.Micro v2.0
- [x] Adapt code to changes in Caliburn.Micro
- [x] Remove WinRT and SL projects from the solution
- [x] Convert project to PCL
- [x] Update NuGet package definition