Real XML-based XPath for iOS [Not ready for merge]

Lots going on here. This is currently blocked until we get https://github.com/michaelkourlas/node-js2xmlparser/pull/10 merged and a release cut for that project, so don't merge yet.

Basically, we're deprecating the old iOS xpath strategy, and in the meantime creating a new strategy "-real xpath" which uses the new way of creating an XML doc from the iOS hierarchy.

Please review, this will be ready to merge once I've updated package.json with the new js2xmlparser rev (or once we've decided to fork it).