Using attr reader internally

This is a subtle change. When you define an `attr_reader` you are essentially defining the following:

```ruby
class MicroBlogger

  def client
    @client
  end
  # ...
end
```

So you can use `client` within the class instead of `@client`. It may seem like a small change to not have the `@` character repeated everywhere, but it is a good way because it now allows you to have one level of indirection if you decide later you need to set up something before accessing the client variable.