Feature/migrate to rabl

Adds rabl/multijson/oj to get some more speed out of the json-rendering. The first view that was converted is the  api/v2/planning_elements/index.api.rabl-view that is used to render the data for timelines. 

The old api-spec was very meager, it basically only tested, that the view is implemented in a certain way. We're now mainly testing the json-version of the api using [json_spec]:https://github.com/collectiveidea/json_spec - it currently contains only some very rough tests: Next step is to check for breaks in timelines/add-ins to get some more cases we need to test. 

If this way of doing the API-views is ok for everybody, we can then proceed with migrating the other views too.