Implement json closer to BBT

[koppor]

(as discussed in the call) - contributes to #1

Just a note: We do not use eta for JabMap, because we do not want to have a whole JavaScript templating engine shipped with JabRef. We can work on this if users want to, but for now, we should "hard code" a "good enough" JSON. Currently, our only consumer is JabMap - and we should create the subset suitable for JabMap (and still be compatbile with BBT)

[palukku]

Added the simple-json format which can be accessed by adding the format=simple-json to the url
http://localhost:23119/better-bibtex/cayw?format=simple-json

After selecting you get a json like this:

[
  {
    "id": 554470445221005466,
    "citationKey": "Ding_2006"
  },
  {
    "id": -4831756210195141935,
    "citationKey": "Cooper_2007"
  }
]