Convenience operations.
Add PrefixMap.isEmpty, PrefixMap.size, PrefixMap.putAll(Map<String, String>)
Add PrefixMapFactory.create(Map<String, String>)

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1443363 13f79535-47bb-0310-9956-ffa450edef68