Added strong parameters to EntriesController.

The format of the parameters has changed a bit for easier integration with this security feature.

BTW I don't like the least bit the syntax that has to be used to allow a param of type array to be accepted by the whitelist.
Added strong parameters security to FeedsController and FoldersController.

The structure of the JSON sent via AJAX to these controllers has been simplified a bit. Only really needed parameters are sent, instead of serializing the whole form to POST it. Also a hidden field in a form has been removed, its value can be included more easily in the AJAX POST with coffeescript code.