Remove FINN-specific config and make code more reusable for others

Have replaced the com (post-message stuff) with a new module, cross-domain-events. That was the last bit to split the gardr code into [gardr-host](https://github.com/gardr/host/) script (parent page) and the [gardr-ext](https://github.com/gardr/ext/) script (inside the iframe).

So now it's possible to require them as regular CommonJS modules from npm. Which makes it possible to register plugins (in the future), and use browserify or similar tool to create custom bundles for your site depending on your needs.

After moving out the main logic of this project, we don't have a need for Grunt or most of the other complicating parts. So cleaning up and making this project into more of a sample project on how to use [gardr-host](https://github.com/gardr/host/) and [gardr-ext](https://github.com/gardr/ext/).