Further refactoring around PrefixMap

Pull more common functionality up into PrefixMapBase, rename PrefixMap2 to PrefixMapExtended and rewrite as a true wrapper class.

Add test classes to exercise PrefixMapExtended

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439993 13f79535-47bb-0310-9956-ffa450edef68