Check for gnuplot before collecting data.

This should address issue:
Check for gnuplot before running - ID: 3062202
http://sourceforge.net/tracker/?func=detail&aid=3062202&group_id=203965&atid=987711

Signed-off-by: Sven van Haastregt <svhaastr@liacs.nl>

gitstats

@@ -1368,6 +1368,10 @@ Default config values:
 			print 'FATAL: Output path is not a directory or does not exist'
 			sys.exit(1)
 
+		if not getgnuplotversion():
+			print 'gnuplot not found'
+			sys.exit(1)
+
 		print 'Output path: %s' % outputpath
 		cachefile = os.path.join(outputpath, 'gitstats.cache')