Variables and lists (#187)

* Import lists and variables from SB2

* Switch to Variable and List objects

* Add Clone.lookupOrCreateVariable, Clone.getVariable, Clone.setVariable

* Add (get, set, change) variable blocks.

* Copy variables and lists on clone instantiation

* Move variable options closer to blocks

* Add list primitives

* Move variable and lists storage to `Target` instead of `Clone`

* Move _computeIndex to a Cast function

* Rename `getList` -> `getListAsString`

* Renames renames

* Remove extra check in Cast.isNaN

src/engine/variable.js

@@ -0,0 +1,18 @@
+/**
+ * @fileoverview
+ * Object representing a Scratch variable.
+ */
+
+/**
+ * @param {!string} name Name of the variable.
+ * @param {(string|Number)} value Value of the variable.
+ * @param {boolean} isCloud Whether the variable is stored in the cloud.
+ * @constructor
+ */
+function Variable (name, value, isCloud) {
+    this.name = name;
+    this.value = value;
+    this.isCloud = isCloud;
+}
+
+module.exports = Variable;