Use RapidXML for XML parsing

[rkday]

Rather than the occasionally buggy custom XML parser SIPp currently uses, we should investigate moving to RapidXML (http://rapidxml.sourceforge.net/).

[rkday]

As RapidXML hasn't been updated since 2009, this is probably not sensible.

[lemenkov]

We'd better to reuse some proven and popular XML library but certainly not this one.