Needed for node-gyp v2.0.0
* This commit makes it possible to create XML commment nodes in nodejs. * A new XmlComment class is introduced which exports a method text() to get and set the comment content. * This patch also includes tests to assert comment creation and the getting and setting of comment content through the text() function.
Essentially the same setup as the `wscript`. It uses the `xml2-config` command to get the necessary -l and -I flags to link to the system libxml2. Therefore, there's no Windows support yet. That will require bundling libxml2 for Windows and statically linking to that, which is something for down the road.