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.
That would look nicer in this class but would make tests know about actions, which they currently don't. As a part of these changes, I have removed all knowledge of the Command structure (which is used to execute tests) from the tests themselves. 

One compromise might be to give Test a property called AttributeProvider. For TestMethod, this would be a MethodInfo, for fixtures a Type and for TestAssembly an Assembly. However, the code below would then still need to test the AttributeProvider to see if it's a Type.

Probably, the future way around this is to review ActionHelper, which I have pretty much used intact from NUnit 2.6.