Don't use localization when writing events with SimpleGcWriter

PrintWriter.printf() uses the default locale of the system for formatting unless a locale is specified.
This affects e.g. numbers in locales that uses a comma instead of a period as decimal separator (and causes a failure in SimpleGcWriterTest when run on a system with Swedish locale).
The fix is to supply null as the locale since this will format without localization (according to the Javadoc for PrintWriter.printf()).
Avoid null cast and replace println() with "%n" in printf().
add unittest for localization independency (issue #50)