Parse XML asynchroneously

[gagern]

This is a spin-off from #30 (comment).

When passing a callback function to the parse or apply function, they perform their operation asynchroneously. However, when one of the arguments is a string which has to be parsed as an XML document, that parsing happens synchroneously using libxmljs.parseXml. It might be more consistent to replace those synchroneous calls with libxml.Document.fromXmlAsync(buffer, callback) which operates asynchroneously. Or with something similar which allows us to fix the encoding so it doesn't get auto-detected from the file header.