Fix problem with webpack build for browser

webpack/webpack#744

Author: Ohar

package.json

@@ -20,5 +20,9 @@
     "lib": "./lib",
     "example": "./example"
   },
-  "main": "./lib/XMLHttpRequest.js"
+  "main": "./lib/XMLHttpRequest.js",
+  "browser": {
+    "fs": false,
+    "child_process": false
+  }
 }