Update board.js to possibly have ERASE CANVAS setting, and fix google chrome errors

Thought it needed a "clear all" selection for the  whiteboard, and fix the google chrome issue when you write a line halfway through

It should work, but I havent changed any more of your files besides this one.

it includes a new var of erasecanvas and erasecanvasBuffer with a px width of 600px (i believe) so one click on the grid would wipe it clean.