Add support for Item#data serialization.

src/core/Base.js

@@ -59,6 +59,14 @@ this.Base = Base.inject(/** @lends Base# */{
 		return Base.exportJson(this, options);
 	},
 
+	// To support JSON.stringify:
+	toJSON: function() {
+		// TODO: We should probably have exportJson() / importJson() deal with
+		// objects rather than strings too, so we can just delegate to those
+		// here.
+		return Base.serialize(this);
+	},
+
 	/**
 	 * #_set() is part of the mechanism for constructors which take one object
 	 * literal describing all the properties to be set on the created instance.

src/item/Item.js

@@ -45,7 +45,8 @@ var Item = this.Item = Base.extend(Callback, {
 		blendMode: 'normal',
 		opacity: 1,
 		guide: false,
-		clipMask: false
+		clipMask: false,
+		data: {}
 	},
 
 	initialize: function(point) {
@@ -142,15 +143,16 @@ var Item = this.Item = Base.extend(Callback, {
 			for (var key in fields) {
 				var value = that[key];
 				if (!Base.equals(value, fields[key]))
-					props[key] = Base.serialize(value, options, compact,
-							dictionary);
+					props[key] = Base.serialize(value, options,
+							// Do not use compact mode for data
+							compact && key !== 'data', dictionary);
 			}
 		}
 
 		// Serialize fields that this Item subclass defines first
 		serialize(this._serializeFields, true);
 		// Serialize style fields, but only if they differ from defaults.
-		// Use no compacting there, since colors always need their type
+		// Do not use compact mode here, since colors always need their type
 		// identifiers.
 		// Do not serialize styles on Groups and Layers, since they just unify
 		// their children's own styles.