refs 164 - more mingw updates, windows version now calls both gnuplot and wgnuplot. Firstly a gnuplot test file is produced, which sets the terminal to unknown so no plots are actually shown, then gnuplot is used to run this test file. This is done as gnuplot actually has an stderr output to show the user of any errors. If no errors are present the proper plot file is run using wgnuplot which is spawned in a new process so that it doesn't block the original process, so now users can have multiple plots open at once as is done on OSX/UNIX with x11