Add ability to stream XML in chunks

[nikku]

Implement Parser#write and Parser#close to enable streaming of XML nodes in chunks:

var parser = new Parser();

parser.on('openTag', ...);

// stream input data in chunks
parser
  .write('<xml>')
  .write('<child />')
  .write('</xml>')
  .close();

This allows the parser to handle huge XML documents in a memory efficient way. It allows for asynchronous step by step parsing, too.