title: Alter existing model with a Redmine plugin
body: <p>The Redmine plugin tutorials explain how to wrap core models but what I need is to add another column to the journals table. 
  I need a boolean field inserted in the journals model. Creating another model with a 'belongs_to :journal' relation seems like an overkill.
  Can this be done with a plugin?
  I should note that I am a rails newbie.</p>