API comparison

Initial commit for the API comparison script. 

A test script has been added that executes a combined search between all the specified API's in the `tests.json` file and outputs the results. This allows us to compare the returned results and check whether or not they match the expected results.
You're absolutely right @timdegroote , @ucamhal also made the same comment on a previous PR. This is currently put **on hold** since we are focussing on the quality and reliability of the external API's. Once this has been solved and decisions are made we can continue development on the libraries gateway website itself. The logic in the Aquabrowser API will definitely undergo some changes, where specific functionality will be isolated and have its own method. That's why I also introduced the usage of **promises** in the comparison script, which allows us to get rid of the **callback hell**. This will also be introduced in the current codebase of libraries gateway.

This PR only contains some code to compare results from all the external API's listed. It doesn't really use the internal code from our Aquabrowser and Summon API's, except for the authentication. In the comparison script, we pass raw queries to the external API's and export these to a json file, without massaging the data. So we don't need the facets, suggestions, etc... 

So for now, I'd rather concentrate on the search result quality and tackle this once we continue development on libraries gateway. But feel free to create a separate ticket for this, so we definitely don't lose sight on this one.