Going in for the operation... (Create Migrations operations based on IndexAttributes)

This change updates the model differ to understand index annotations in the store model and create appropriate create and drop operations for the indexes when they are added to the model, removed from the model, or changed in the model.
Cleanup build system

Changes:
* Remove OpenCover
* Unify SkipStrongNames targets
* Use NuGet.exe restore
* Simplify xUnit.net targets
* Move NuGet package sources to NuGet.config