Add target to /api/mentions endpoint results

Proposed to fix https://github.com/voxpelli/webpage-webmentions/issues/12.
Include all mention targets in a single entry

To avoid returning duplicate entries.

Fixes #12, #13
0.3.11
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.