Align NUnitDomainModel with NUnit documentation

[andersosthus]

This PR changes two things:

• The default value for NUnitDomainModel is changed from SingleDomainModel to DefaultDomainModel. In addition DefaultDomainModel is changed from None to "", causing the parameter -domain to be excluded in the call to NUnit. This in effect reverts Nunit Domain Fix per #855 #883, as it caused regressions in exsiting build scripts (Failing NUnit tests in v4.0.2 #896) that depends on NUnit selecting the App Domain Model itself, as specified in the documentation (http://www.nunit.org/index.php?p=consoleCommandLine&r=2.5).
• Added a new value for NUnitDomainModel, NoDomainModel, which adds -domain:None to the command line. This gives the same effect as the change done in Nunit Domain Fix per #855 #883, but it's now an opt-in, and not a default value.

In effect:

• App domain model should by default now be as before Nunit Domain Fix per #855 #883.
• It is now possible to specify all of the supported app domain models supported by NUnit:
  • NoDomainModel
  • SingleDomainModel
  • MultipleDomainModel
• For Exception System.InvalidCastException thrown when running NUnit and FsCheck tests #855 and NUnit/FsCheck.NUnit does not work with mono fscheck/FsCheck#115, they should now specify Domain = NoDomainModel to get the intended behavior (-domain:None)

Also included tests to verify argument generation for NUnitDomainModel.

[forki]

@TheAngryByrd could you please review this? Thanks

[TheAngryByrd]

Looks good to me!

[forki]

Thanks a lot

[forki]

I still have an issue with sourcelink. After that I can release this. Thanks everybody