Add the Leviathan pow(?x, ?power) function (JENA-507)

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1516515 13f79535-47bb-0310-9956-ffa450edef68
Updated tests for Leviathan functions (JENA-507)

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1516516 13f79535-47bb-0310-9956-ffa450edef68