Support for tracking ActiveRecord models allocations

Hi Dejan,

these patches add tracking for AR models allocations by adding an after_initialize hook into ActiveRecord::Base. This hook then triggers ActiveSupport notifications and a separate panel displays them [(screenshot)](http://balazs.kutilovi.cz/tmp/rails_panel/rails_panel.png). At the moment, it's just a proof of concept, so no tests and unpolished code, etc..

Do you think you'd be interested in this kind of functionality, i.e. extensions + new event notifications being added to rails_panel, or would you prefer to keep the rails_panel only for displaying events that are already part of rails?