Implemented basic external "write-file" function
* accepts a path parameter and the content to be written
* removed target parameter form FunctionTransform
** transformation result is now returned as a string
*** nodeset return value is planned
** e.g. writing the result to the fs is optional and has to be achieved using FunctionWriteFile
* changed "transform" test case accordingly
* this marks a paradigm shift and is the continuation of the changes described in 741a70f
** InputXSLT now also implements a output function
* added basic io error handling to FunctionReadFile