Set output.globalObject option in Webpack config

Fixes #1126

This fixes a reference error when using the build in a WebWorker.

This is a workaround taken from webpack/webpack#6642 (comment)
and should be replaced by `target: "universal" when webpack/webpack#6525
has been implemented.

Author: galfert

webpack.config.js

@@ -22,7 +22,8 @@ module.exports = {
     // global export name if needed
     library: 'RemoteStorage',
     libraryTarget: 'umd',
-    umdNamedDefine: true
+    umdNamedDefine: true,
+    globalObject: 'this'
   },
   // the only external dependecy is xmlhttprequest because it is
   // different in browser and in node env so user has to manage with that