Add support for NUnit tests defined via attributes

[dgrunwald]

SD-1733, originally created on 9/28/2010 20:42:30 by Matt Ward

See:

1. http://community.sharpdevelop.net/forums/t/10724.aspx
2. http://nunit.org/index.php?p=testCase&r=2.5.7
   http://nunit.org/index.php?p=testFixture&r=2.5.7
   http://www.nunit.org/index.php?p=testCaseSource&r=2.5