Support for instance variables

Support for instance variables.  Following is sample.  Let me know if you have any questions.

The sample class
```ruby
class Person
  def initialize
    @retired = false
  end
end
```

Without the support:
```ruby
describe '.initialize' do
  subject { Person.new }
  it('will not be retired') { expect(subject.__send__('retired')).to eq false }
end
```

With the support
```ruby
describe '.initialize' do
  subject { Person.new }
  its('@retired') { is_expected.to eq false }
end
```