add sideEffects: false to package.json

Author: rpearce

package.json

@@ -32,6 +32,7 @@
     "README.md",
     "src/"
   ],
+  "sideEffects": false,
   "scripts": {
     "build": "npm run build:js && npm run build:docs",
     "build:docs": "browserify example/app.js -o docs/app.js -t [ babelify ]",