Remove dependency of NUnit from AllureCSharpCommon.

AllureCSharpeCommon is common library for integrate Allure with C#. But it has a dependency with NUnit. So user (like me) who is not using NUnit in there project, bound to use NUnit.
This pull request removed dependency of NUnit from Core framework(AllureCSharpeCommon). So that user can use it with any testing Framework like : MSTest, XUnit etc.