added UnitTests for org.scaloid.common.Helpers

Please let me send PR for UnitTests of org.scaloid.common.Helpers .

Of note,this doesn't include any UnitTest for org.scaloid.common.MediaHelpers since Robolectric seems not to support tests for RingtoneManager (and I couldn't find any other suitable test framework... :-( ).

If any, please let me know.