Moved DOM document construction logic into local functions
* this change makes the underlying similarity between all currently implemented external functions obvious
* they more or less only differ in their document construction logic
** i.e. there is a possibility for a second layer of abstraction between the classes derived from xalan::Function and the actual functionality
* added boost::filesystem::path taking overload of the _iterate_ member method to FilesystemGuard
** this overload functions as the _master_ overload that is called by all other overloads which merely perform type conversion and path resolution
** this overload doesn't perform path resolution