Bugfix/fix continous streaming

fixes #56 by following the proposed solution. now the first data available from the device buffer gets parsed immediately. 

The [qt documentation about incremental parsing](http://qt-project.org/doc/qt-5/QXmlStreamReader.html#incremental-parsing) misses some best practice how to write new data to the device during parsing. However, this solution uses an event loop which  waits for new data once the `QXmlStreamReader::PrematureEndOfDocumentError` occurs, writes it to the device after `readyRead()` signal got emitted and resumes parsing.