Implement Item#data cloning.

Closes #340.

src/item/Item.js

@@ -1313,6 +1313,8 @@ var Item = Base.extend(Callback, /** @lends Item# */{
 		}
 		// Use Matrix#initialize to easily copy over values.
 		copy._matrix.initialize(this._matrix);
+		// Copy over _data as well.
+		copy._data = this._data ? Base.clone(this._data) : null;
 		// Copy over the selection state, use setSelected so the item
 		// is also added to Project#selectedItems if it is selected.
 		copy.setSelected(this._selected);

src/style/Color.js

@@ -760,6 +760,8 @@ var Color = Base.extend(new function() {
 		 *
 		 * @return {Color} a copy of the color object
 		 */
+		// NOTE: There is no need to actually implement this, since it's the
+		// same as Base#clone()
 
 		/**
 		 * {@grouptitle String Representations}