Direct test execution fix

Corrects a bug that occurs when a test script is dot-sourced into the global scope.  In that situation, mocks can wind up being executed by modules that were not intended to see the mock (including Pester itself.)
Merge branch 'dlwyatt-GlobalMockFix' into Beta
ParameterFilter fixes

The script block passed to the -ParameterFilter parameters on Mock and Assert-MockCalled is now executed in the test script's scope.  To prevent errors when the parameter filter is being evaluated in some cases, Strict Mode is disabled within that block (but left alone otherwise).s
Merge branch 'dlwyatt-ParameterFilterFixes' into Beta
NUnit OutputXML updated

The output XML used templates to output the nUnit xml report, that did not support using special
characters in Describe and It descriptions. I replaced it with XmlTextWriter that does all the
escaping automatically. Also the nUnit report was not compliant with the scheme. It now has single
test-suite root name by the Path (is there better option?). Scheme is not verified as well as the
special characters.

Fixing #122