Made TraceSource return param

To allow TraceInformation("Before bad").TraceError("Bad").Flush();

My tests show that "return" instead of void does not adds performance overhead if return object not used and method not inlined. I can be wrong, but made tests https://github.com/OpenSharp/NStopwatch/blob/master/src/Invocation.Tests/InvocationsTests.cs to have some guaranty.