Installs Akka.TestKit.TestEventListener as default logger for TestKit

The default `Akka.Event.DefaultLogger` is replaced by `Akka.TestKit.TestEventListener` as the default logger for the system created by TestKit.

This logger behaves exactly like `DefaultLogger`, but makes it possible to use EventFiltering. If no filter is specified it logs to StdOut just like `DefaultLogger`.

Discussed in issue #323