Leverage NuGet restore to install development dependencies

From https://github.com/libgit2/libgit2sharp/issues/739#issuecomment-63465804
- Upgrade NuGet binary to 2.8 (https://nuget.codeplex.com/workitem/3332)
- Drop xUnit and Moq binaries from the repo
- Drop the references from the projects
- Install those from NuGet
- Trigger the restore on `BeforeBuild` event
- Fix the path to the xunit runner in `CI/build.msbuild`
