Changed system-ID of primary transformation to working directory
* this change was needed so that one is able to apply transformations located in e.g. a central directory as if they where located in the working directory
** otherwise e.g. the BuildXSLT transformation would have to be copied into every project that requires it
* implemented StreamInputSource wrapper for boost::filesystem::ifstream and xalan::XSLTInputSource
** this is needed because the boost::filesystem::ifstream instance has to exist as long as the xalan::XSLTInputSource it is bound to
** i.e. a simple helper function to instantiate the source and set the working directory would have not been sufficient