fix issue where using the eraser tool in firefox clears entire sketch

see: http://stackoverflow.com/questions/3328906/erasing-in-html5-canvas

"Note that firefox 3.6/4.0 implement 'copy' by erasing the entire background first. The w3c docs aren't clear as to what should happen here. Chrome (webkit?) interprets the specs as 'only where pixels are actually drawn', for example the result of a stroke()."
Merge pull request #4 from kmstrong/master

Fix for eraser bug in firefox