Add support for processing instructions and comments

Currently xmlToJSON only support element, cdata, and text nodes.

This is related to issue #21 . Without significantly altering the format of the JSON object, there is not a simple approach to maintaining ordering for same-key nodes that are interspersed with other nodes. Without strict ordering, context is lost for items such as comments and processing instructions. And in the end, you really should just parse the xml if you need that level of fidelity :)

[javadev]

https://github.com/javadev/xml-to-json will help