Allow configuration of iframes' javascript files

This PR is the same direction as #155. With this change, we could massively speed up our entire test suite. Around a factor of 10 to 20.

This change is fully compatible with the existing source as it only allows the linked javascript files in the `iframe.html.erb` to be configured.

By including a `spec_helper.js` file together with the `chai` and `konacha/iframe` we massively reduced the number of HTTP requests. Also sprockets only needs to compile the specs itself for every iframe as we can omit the `\\= require "spec_helper"` at the beginning of the spec files.