Move to .NET4 and implement new add in manager

Use an add in manager based on MEF.
Refactor projects

Rearrange the project layouts to allow for a cleaner layout when we
implement the task tracker abstractions.
Create Models/ Abstraction.

Resolves #7
Add models/ abstraction of Tasks.
Add sample to Main Window

Now that we have some models, use them to drive a sample UI using the
stub task tracker.