no-PTFE brass hot-end

This is a hot-end modification for the MK4 extruder that removes the PTFE, which is prone to melting and swelling, from the hot end. It is a derivitive of lampmaker's all-steel hot end ( http://www.thingiverse.com/thing:3452 ), where this design uses the brass parts from a failed MK4 hot-end plus some easily made aluminum fins.