Expose "Date Style"... "Use System Default" only when FuzzyMoment is off.

If the Date Style "Use System Default" is selected and FuzzyMoments is clicked on, then an appropriate Date Style is chosen from explicit defaults.  This change prevents issues where moments() cannot interpret or guess locale date formats.
Merge pull request #588 from JackDandy/FuzzyMoments

Fuzzy Moments Update