Add :accessible option to Associations for allowing mass assignments using hash. [#474 state:resolved]

Allows nested Hashes (i.e. from nested forms) to hydrate the appropriate
ActiveRecord models.

class Post < ActiveRecord::Base
  belongs_to :author,   :accessible => true
  has_many   :comments, :accessible => true
end

post = Post.create({
  :title    => 'Accessible Attributes',
  :author   => { :name => 'David Dollar' },
  :comments => [
    { :body => 'First Post!' },
    { :body => 'Nested Hashes are great!' }
  ]
})

post.comments << { :body => 'Another Comment' }
Signed-off-by: NPratik Naik <pratiknaik@gmail.com>