Guard from localization issue in WP(8) REAL result to Javascript

@briancalvillo suggested a possible fix using
System.Globalization.CultureInfo in #123. This _could_ be needed on
devices with European localization. Using CultureInfo.InvariantCulture
in both Convert.ToDouble() and String.Format() for REAL (Double) results
to be extra-safe.
Merge branch 'master-src' into master-rc: guard from possible localization issue in WP(8) REAL result to Javascript (ref: #123)