Fix for Issue #163 - Add --trace option to NUnitLite

This is pretty straightforward. Beyond adding the command line option and the tests for it, I had to modify TextUI to initialize the internal trace. This meant that I needed to move the creation of the runner from the constructor into the Execute method after the initialization of the internal trace otherwise several static loggers would be created uninitialized.

The default is logging off. I have tested with and without the command line option and with the unit tests I added.
The name is confusing... why "Invalid"?