Addes Clear() to TiXmlPrinter

[GoogleCodeExporter]

Am I doing something wrong? I could not manage  a way to serialize elements 
with << operator. 
Considereing I'm doing this task a lot, i found necessary to add a function
TiXmlPrinter::Clear() because of my needs. It just clear the buffer string and 
allow to reuse the 
object.

void Clear() {buffer.clear();}

Original issue reported on code.google.com by tnnogue...@gmail.com on 15 Jan 2009 at 8:58