Issue 75 - Async tests should be supported for projects that target .NET 4.0

**PLEASE MERGE**

Work here is mostly completed and I'm submitting for review. This extends the current support for async tests to .NET 4.0, which one can achieved using a Microsoft package distributed via NuGet.

Per-se the feature was already there once #64 was done, this PR is mostly about testing it. This implies referencing the Microsoft package from our .NET 4.0 test projects and running all the tests that before were exclusively for 4.5.

Unfortunately automatic package restore via NuGet works horribly cross-platform, therefore the only reasonable option I found to get those binaries into our test projects was to add them to the repository. I do not like this, but keep in mind that they are only referenced by NUnit's test projects, not NUnit itself. If you can think about a better way feel free to share.

Most files added/changed here are coming from those NuGet packages, the rest is mainly minor changes to get async tests into .NET 4.0 test projects.