initial commit for conversations rewrite

This is my latest brain-storming...

* introducing new (JSON-only) private API for backbone.js requests - utilizing a vendor-specific `Accept` header for the request
* using a mounted Sinatra app to keep overhead as small as possible - the Rack stack will have been set up by Rails, only the test env requires some manual Rack stacking
* making presenters more versatile ( - `#base_hash` mustn't require associated records, `#full_hash` may include them)

TODO:
* JS
* UI
(duh...)

please comment and brainstorm with me, whether this makes any sense...