Updated CHANGES.md for b13

Author: pjcozzi

CHANGES.md

@@ -4,7 +4,7 @@ Change Log
 Beta Releases
 -------------
 
-### b13 - 2013-xx-xx
+### b13 - 2013-02-01
 * Breaking changes:
    * The combined `Cesium.js` file and other required files are now created in `Build/Cesium` and `Build/CesiumUnminified` folders.
    * The Web Worker files needed when using the combined `Cesium.js` file are now in a `Workers` subdirectory.