Implement unique ids, and fix issue with Base.deserialize()

Gradient and Symbol were overriding each other in the dictionary, and dictionary entries could not contain references to other dictionary entries.

Closes #690

src/util/UID.js

@@ -0,0 +1,30 @@
+/*
+ * Paper.js - The Swiss Army Knife of Vector Graphics Scripting.
+ * http://paperjs.org/
+ *
+ * Copyright (c) 2011 - 2014, Juerg Lehni & Jonathan Puckey
+ * http://scratchdisk.com/ & http://jonathanpuckey.com/
+ *
+ * Distributed under the MIT license. See LICENSE file for details.
+ *
+ * All rights reserved.
+ */
+
+/**
+ * @name UID
+ * @namespace
+ * @private
+ */
+var UID = {
+    _id: 1,
+
+    /**
+     * Returns the next unique id.
+     * @method get
+     * @return {Number} The next unique id
+     * @static
+     **/
+    get: function() {
+        return this._id++;
+    }
+};