1.0 May 23, 2014.

1.1 Dec 19, 2014

Feb 2 2015: added resetTurtle.

July 25, 2015: imprintLine was writing on the display.  Made it write
on screenBG.

July 12, 2016: Put a new version with compiler option changed from
-std=c++0x to -std=c++11 because the former has been deprecated.

June 7 comparison on line 117 of canvas.cpp changed to first coerce gc
to long int.
