Instantiate observers via Railtie, and run load hooks

I spent some time figuring out how to cleanly use the
ActiveResource observer support in a Rails app, finding surprisingly
little documentation beyond ActiveModel::Observer itself, arriving at
the need for this essential boilerplate in an initializer:

    ActiveResource::Base.observers = # ...
    ActiveResource::Base.instantiate_observers

This change packages it up as neatly as ActiveRecord observers in Rails:

    # In config/application.rb
    config.active_resource.observers = [:post_observer, :comment_observer]

The Railtie will instantiate observers automatically, and reload them
for each request in development just like rails-observers does for
ActiveRecord.
Merge pull request #109 from DisruptionCorporation/observers-railtie-and-load-hooks

Instantiate observers via Railtie, and run load hooks