Classes: use attributes directly instead of call accessors methods.

Hello!

I added a few rules for use of `attr`-family methods inside a class definition.

In example `name` is equivalent of `self.name` and this is a complete method call.

`@name` is more readable, and faster in use.

You can simple test it something like this:

```ruby
class TestClass
  attr_reader :name

  def initialize
    @name = 'test_name'
  end

  def test1
    t = Time.now
    100_000_000.times{ @name }
    Time.now - t
  end

  def test2
    t = Time.now
    100_000_000.times{ name }
    Time.now - t
  end
end

x = TestClass.new
puts x.test1
puts x.test2
```

Moreover you can never shade `@name` of a some local variable. :smirk: