Add JSON end-points for Biohackathons and authors

[pjotrp]

We currently show a full JSON output on http://biohackrxiv.thebird.nl/list.json. We should have simple end-points that render:

/list.json (all)
/#{event}.json
/#{author}.json
/#{group}.json

to facilitate embedding in web pages. Note that list.json may get large in time --- or we'll paginate.