CanCan like Abilities

[sebastianhoitz]

Salad should introduce Rails-like CanCan abilities: https://github.com/ryanb/cancan

This should have a separate class that defines what a user can do.
It can get a little hard because we have to do this asynchronous,
but something like this should be possible:

class Abilities
  @initialize: (user) ->

    # register an ability for the user on articles
    @can "article", (done) ->
      user.getForums().all (err, forums) ->
        forumIds = (forum.get("id") for forum in forums)
        conditions =
          forumId: forumIds
        # this registers additional conditions the scope has to contain to secure that the user can only edit "articles", that are in a forum the user has access to.
        done err, conditions

The controllers should be able to implement a cancan mixin, that hooks via @beforeAction filters into the flow of dispatching a request and can limit the scope that way.

Relevant pieces to instrument in the controller are:

• @findResource
• `@findParent``
• @scoped