enable unittests for net45 - #37

• Wouldn't it refuse to load the assemblies because the test project isn't signed? I don't think we can disable signing entirely, as the nuget packages should be signed at least. I could go with not signing in Debug if it's a problem.
  • In fact.. How come the runner is signed - i thought strong name signed assemblies could only load other SN signed assemblies ... An unsigned assembly should never have a problem loading signed assemblies though.
• Great optimization reg. nuget on debug
• If we support net4x tests, they should be run in CI. I assume AV's image (VS2017 I think) should support both core and net4x in the same go. So it shouldn't be too hard - I might look at that one.

To be clear: I only disabled PublicSign - SignAssembly is still set to true. So the assemblies are still signed.
I don't have enough knowledge about signing, to tell the exact difference between SignAssembly and PublicSign and which problems may arise from each option.

The condition was actually stolen from an asp.net core example (or the MS docs), but I have no idea, what the reason behind this condition is since I was not able to find any useful documentation.

I've rebased onto #60 and also enabled the unit tests for net45 in appveyor.

There is still an issue with parallel test execution on net45 (thus the failing build) - I will look into this.

The last commit disables parallel test execution, which fixes the build. Since the whole discutils project is not thread safe, I guess there is no benefit in trying to make the collections of registered modules thread safe.

Thanks!