Cleaned up join util.
Added empty XSL assertion.
Ensure that internal elements are copied during a join.
Reworte xpath join so that or XPath operations are not joined.

There were bugs in the way we were joining things, when oring
XPaths. Removing these for now, will look at including later.
Added validaton tests to confirm xpath join fix.
Added builder tests to confirm xpath join fix.
Merge pull request #115 from InABlender/xpath_join_fix

Problem with joinXPath optimization