fix compatibility issues caused by IE
using paperjs to render whole atlas editor, implemented selection and dragging.
use closure function instead of private function in AtlasEditor
use canvas to draw whole work space
small refactoring
can drag workspace by right button
fix compile error
fix compatibility problem about mouse event for firefox and ie
add zoom in and out
update
extract common canvas operations to WorkSpace
refactoring
change zoom pivot to viewport center
refactoring
merged from paperjs zoom