Exit code fix

Hi Giorgio,
we noticed that your `WrapperRunner` always returns 0 as exit code.

So using giorgiosironi/paratast in a CI environment such as Jenkins doesn't work because every build will be marked as SUCCESS even if there is an error.

Here we fixed `WrapperRunner` in order to return proper exit codes according to the test results.

We've been using it for more than a week without problems.

Bye.