Fix text formatting problems for non en-US systems

The output in the nUnit report should be culture agnostic. So in all
functions that export data. casting to [string] has to be used instead
of ToString() (unless you specify the proper culture settings to be
used, which is more explicit but does not work in Parameter
specification)
Fix the ItImpl parameters

Fixed the ItImpl parameters to make it work correctly with the Skipped
and Pending. I also set the ScriptBlock to empty scriptblock after
validating all parameters. That way the Invoke-Test function interface
is kept strict (it requires a ScriptBlock) while the desired
functionality is kept.  I am not sure if this is hundred percent
correct, because I am not sure where the boundary between Pester and
'Common Testing Framework' should be and what these should and should
not implement.