Non-model backed sync components

[ajb]

I've run into some instances where I want to sync something that's not tied to a particular resource, such as a count of unread notifications (#191), or a collection of other resources (#112).

It would be interesting if we could remove the hard dependency on an AR model, and do something like:

Views:

<%= sync 'unread_notifications_counter', scope: current_user.id, count: current_user.calc_unread_notifications_count %>

<span class='badge'><%= @count %></span>

Worker:

def perform
  send_notifications # or whatever
  sync_update 'unread_notifications_counter', scope: @user.id, count: @user.calc_unread_notifications_count
end