Detecting attributes from model

Hi, I have implemented automatic attributes detection from model, what do you think about it?

Let's have a model

```ruby
class Book
  include Virtus.model

  attribute :title
  attribute :price
  attribute :description
end

# or

class Book
  attr_accessor :title, :price, :description

  def self.attributes
    [:title, :price, :description]
  end
end
```

After this change you can write mapper

```ruby
class BookMapper < Perpetuity::Mapper
  map Book
  detect_attributes
end
```

Instead of 

```ruby
class BookMapper < Perpetuity::Mapper
  map Book
  attribute :title
  attribute :price
  attribute :description
end
```

It can be extended to detect also attribute types and other attribute options.