Change default script scope to global.

With Microsoft's PowerShell the following:

    Pipeline.Commands.AddScript(script);

is equivalent to:

    Pipeline.Commands.AddScript(script, useLocalScope: false);

Added a reference test for check this behaviour.
Fix broken assert

First, it was asserting the wrong expression, which had no value. Fixed.

Second, Debug.Assert is ignored when NUnit runs in Travis-CI. Added a TraceListener to make it that fail.
Merge pull request #220 from mrward/script-default-scope

Change default script scope to global.