New Padrino generator command for creating observers

I stumbled across this missing feature during writing on my book about the observer section <http://padrinobook.com/book/chapters/login_and_registration.html>. 

You just have to call either `padrino g observer <Name-of-the-observer>` or `padrino-gen observer <Name-of-the-observer>` and it will create a file in `app/models/<name_of_the_observer>.rb`.

So this is basically the command for creating new observers.

In order to use `observers`, you have to add the following in the `Gemfile`:


```ruby
gem 'activerecord', '~> 3.2', :require => 'active_record'
```


As well as activate observers in `app/app.rb`:


```ruby
module Test
  class App < Padrino::Application
    use ActiveRecord::ConnectionAdapters::ConnectionManagement
   ...

    enable :sessions

    # Activating the user_observer
    ActiveRecord::Base.add_observer UserObserver.instance
  end
end
```


I uploaded an example app under <https://github.com/wikimatze/padrino-observer-example-app> so that you can play around with it.


If you are having an observer-file with the right model name, Padrino gives us a nice error message of the form: `ERROR -  NameError - uninitialized constant User:`. If there are any tests missing or I forget to add another case, please let me know.

Bests,

Matthias