Feature actionattributes

This pull request completes the implementation of ActionAttributes to the same level as supported by NUnit 2.6. The ITestAction interface changes to allow for the fact that the attribute can now access the internal representation of a test directly.

Ultimately, it would be desirable to interleave calls to action attributes with corresponding setup/teardown calls. However, in this implementation as in NUnit 2.6, all before actions take place after setup and all after actions before teardown. Further enhancement will be done as part of a new issue.
Here and in several other places, these could be made readonly. I like the syntax as a declaration that the variable will/should only be set in the constructor. Probably not needed for such a small class that you can scan so quickly, but I like to be as explicit as possible in our intentions.