XML::SAX::Expat subclass for non-blocking (incremental) parsing

 Most XML parsers give a callback interface within an encapsulated loop and
 eventually, when the XML source is depleted by the parser, the parse function
 will return.
 .
 Sometimes you don't want the parser to control the loop for you. For example,
 if you need to retrieve your XML in chunks in a funny way, you might need to
 use a kind of iterator to get your XML data.
 .
 This module provides this possibility.