Add a new endpoint, /api/mentions/live, which is an EventSource and therefore gives Instant Gratification to people wanting to know when webmentions have arrived for their sites and urls.
Make HTTPS a config option
Extended the EventSource functionality

Additions are:

* Works with multiple Node instances thanks to Postgres NOTIFY/LISTEN
* Extended to work with complex combinations of urls and sites in the query arguments
* Fully integrated with the WebMention embed script
* Returns the full WebMention object and maps it as a "mention" event

Fixes #11
Ensured that throttling is deactivated in tests
Added tests for EventSource endpoint
Added CORS-headers for /api/mentions resources
Added a CHANGELOG entry
Merge branch 'pr/14'