XPath API should support third-party indexes for performance

[rolfl]

Some third-party XPath Libraries (like Saxon) can build an index on a document, and as long as the underlying document does not change, the index can be used to improve the performance of XPath queries.

JDOM should be extended to allow users to build these indexes, and to reuse the index if the user knows the underlying JDOM document has not changed between XPath invocations