Support for Rails 4

Add a support for Rails 4.0.0. Basically, just:

  * Add an Appraisal file for this version
  * Move the Announcement model to a generator template to customize
the generated file depending on Rails version
  * Add a call to `attr_accessible` if Rails version < 4.0.0
  * Disable generation of the JavaScript file under this version
  * Fix the generator require statement and add a next_migration_number
class method