let AR::Relation pretty_printed like an Array

[amatsuda]

While pp [model1, model2] looks like this

[#<Model id: 1, created_at: "2012-01-20 14:05:39", updated_at: "2012-01-20 14:05:39">,
 #<Model id: 2, created_at: "2012-01-20 14:05:42", updated_at: "2012-01-20 14:05:42">]

pp Model.scoped prints all AR instances in one line

[#<Model id: 1, created_at: "2012-01-20 14:05:39", updated_at: "2012-01-20 14:05:39">, #<Model id: 2, created_at: "2012-01-20 14:05:42", updated_at: "2012-01-20 14:05:42">]

Added AR::Relation#pretty_print to let PP regard a Relation as an Array of models.