Replace xmldom dep with xmlshim. Remove xpath dep.

[xmlshim](https://github.com/znerol/node-xmlshim) aligns much better with the standard `DOMParser` API then `xmldom`. Allows us to use the same code in  `src/urdf/UrdfModel.js` for both node and browser.

I removed a dependency by hard coding a constant (See https://developer.mozilla.org/docs/XPathResult#Constants)