Adding .NET 4.5 Support

Fixes #384 

Details:

- [x] Introduces the System.Windows.Interactivity v4.5 that comes with Blend and Caliburn.Micro
- [x] adds a new project to build against .NET 4.5 and System.Windows.Interactivity v4.5
- [x] updates nuspec to output correct files

If I can get someone else to test this before I push it in a new alpha that would be sweet.

EDIT: nevermind, I'm impatient. Gonna test it out now.