Report start and end time of test execution

Fix for the following issues, https://github.com/nunit/nunit-framework/issues/31 and https://github.com/nunit/nunit-console/issues/13

See the full comments in https://github.com/nunit/nunit-framework/pull/94 for more information. To keep things simple, let's keep the overall discussion in that pull request and limit comments on this pull request to code comments only.