Remove "pessimistic" from estimated duration string

The computed duration is no more pessimistic, for a few reasons.
- the main one is that it does not take retraction into account
- Z moves are not taken into account either
- arcs are not supported
- the acceleration code seems to estimate acceleration time correctly but is
  wrong for deceleration
So instead of telling the user this estimation is pessimistic, let's just tell
them this is an estimate.

printrun/gcoder.py

@@ -331,7 +331,7 @@ def main():
     print "\tZ: %0.02f - %0.02f (%0.02f)" % (gcode.zmin,gcode.zmax,gcode.height)
     print "Filament used: %0.02fmm" % gcode.filament_length()
     print "Number of layers: %d" % gcode.num_layers()
-    print "Estimated duration (pessimistic): %s" % gcode.estimate_duration()
+    print "Estimated duration: %s" % gcode.estimate_duration()
 
 
 if __name__ == '__main__':

pronterface.py

@@ -1295,7 +1295,7 @@ class PronterWindow(MainWindow, pronsole.pronsole):
         print _("the print goes from %f mm to %f mm in X\nand is %f mm wide\n") % (gcode.xmin, gcode.xmax, gcode.width)
         print _("the print goes from %f mm to %f mm in Y\nand is %f mm wide\n") % (gcode.ymin, gcode.ymax, gcode.depth)
         print _("the print goes from %f mm to %f mm in Z\nand is %f mm high\n") % (gcode.zmin, gcode.zmax, gcode.height)
-        print _("Estimated duration (pessimistic): "), gcode.estimate_duration()
+        print _("Estimated duration: "), gcode.estimate_duration()
         self.gviz.clear()
         self.gwindow.p.clear()
         self.gviz.addfile(gcode)