Dispose of test runners after they run when running in the console

This is the simple fix for issue #308 and nunit/nunit-console#27. I add a `DisposeRunners` setting to the test package in the console and then in the `AggregatingTestRunner`, if it is true, I dispose each runner as soon as it is finished running. I don't dispose after exploring tests, only after running. I think if we run out of memory just exploring tests, then I doubt disposing the runners will help. Anyone think otherwise?

I am not sure how to add unit tests for this. I tested by running in the debugger with multiple test assemblies.