Enable signing of API jars.
Enable signing of API jars.

- TODO: sign POM as well.

hoanganhx86/SeriesGuide,artemnikitin/SeriesGuide,0359xiaodong/SeriesGuide,UweTrottmann/SeriesGuide,r00t-user/SeriesGuide,UweTrottmann/SeriesGuide,epiphany27/SeriesGuide