Indicate package.json 'module' entry has no side effects for WebPack (#72)

Author: rbuckton

package.json

@@ -24,5 +24,6 @@
     "main": "tslib.js",
     "module": "tslib.es6.js",
     "jsnext:main": "tslib.es6.js",
-    "typings": "tslib.d.ts"
+    "typings": "tslib.d.ts",
+    "sideEffects": false
 }