Create IntVal object (related to Issue #17)
Add 'undefinedString' parameter to IntVal
Change Shapes to use IntVal for .x and .y
Change Circle.radius, Shape.line_width, etc to IntVal
Change Item.id, Item.visible and Item.cursorStyle to be enumerable
Bugfix: Group.prototype.moveAndResetOffset not using this[index].x.set()
Rename Shape.offset to Shape.dragOffset (Issue #18)