Fix for disappearing grid if col lower than 1

If the document width is smaller than option.width, this.col gets lower than 1, which makes the grid disappear. 

So, I added a check if this.col is lower than 1, make it equal to 1. There may be a more sophisticated solution, but this fixes the problem really quick. ;)
Merge pull request #21 from falconwhite/master

Fix for disappearing grid if col lower than 1