JENA-768 : Enable using choice of output format.

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1620711 13f79535-47bb-0310-9956-ffa450edef68
Convenience opening of output streams (may be with gzip encoding)

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1620717 13f79535-47bb-0310-9956-ffa450edef68