ApiController changes content-type for all calls

to application/javascript, which is what our browser is expecting.
Added rabl-rails and oj

For better JSON generation!
Added `summary` endpoint

This is for generating a user’s current project’s summary.
We’re using RABL for the JSON.
We also created `checkin_activity` to the Project model, which returns
an array of ints depicting the actions a user took throughout the life
of their project.
Modified the home feed call to use RABL