#94 Throw an informative exception when trying to create an abstract class with public constructor
#94 Address issues raised in pull request
#94 Improve exception message and rename test type
Addressed a couple of Code Analysis warnings.
Moved test of creating abstract class with public

ctor to a separate test file, because I also want to add a test that
demonstrates how to get around the exception. That test will require a
test-specific nested class, so I think it's nicer to have that in a
separate test class, not polluting FixtureTest.cs.
Added a test that demonstrates how to work around

the issue of an abstract class with a public constructor.
Made an exception message more detailed.
Addressed more CA warnings.
Incremented patch version,

indicating that the addition of a more explicit exception message in the
case of abstract classes with public constructors isn't really a new
feature, but rather a refinement of existing behaviour. Some may consider
it a bug fix, although there never really was a bug.