Support for NUnit Attributes on Concordion Test Fixtures

The Java version of Concordion supports several annotations (http://concordion.org/Tutorial.html#advanced). For example the @Unimplemented annotation is used to ignore Concordion tests as long as they are not implemented. A similar concept is available in NUnit through the "Ignore" attribute. To be able to use such NUnit attributes in Concordion-NET the Concordion.Integration.NUnit.Addin.SuiteBuilderAddin was extended.