Feature/for test frameworks

- Moved nUnit fluent assertions out of NSpec and put into a separate project with NSpec.Assertions namespace. Added MSTest fluent assertions to the same project.
- NSpec actions can be used directly in nUnit and MSTest class functions when TestSpecification attribute is applied (and test class inherits from nspec).  This eliminates the need for shim classes in order to use NSpec within Visual Studio or nUnit test runners.