Implemented custom xercesc::DOMDocument deleter
* pointers to xercesc::DOMDocument were manually released
** this is now solved using a custom deleter for the appropriate std::unqiue_ptr template specialization
* added matching factory method to DomDocumentCache
* updated external function implementations accordingly
** "constructDocument" is now expected to return a DomDocumentCache::document_ptr instance
* updated TransformerFacade accordingly
* this change was implemented to get rid of the manual memory management required by xalan / xerces