Update README to use proper quotes for inline code.
Add Module#extended_modules.

Module#extended_modules is analogous to Module#included_modules.
Suggested by http://stackoverflow.com/a/5197277.a
Allow 2 ways to create a model:

Preferred method:

  class User
    include Virtus::ActiveRecord.model
  end

Alterate method:

  class User < Virtus::ActiveRecord::Model
  end