Yielding vs. Instance Evaluating (The Instance Variables issue)

With some tips from:
* http://rubylearning.com/blog/2010/11/30/how-do-i-build-dsls-with-yield-and-instance_eval/
* http://erniemiller.org/2012/06/07/accessing-instance-variables-in-squeel-or-fun-with-bindings-and-instance_eval/

... It was easy to make the best of 2 worlds be availabe for the gem.

Now I can access intance variables outside the block with the new interface.
Below it is a perfectly working code.

```ruby
@my_inst = "My Banner"
SpinningCursor.start do |sc|
  sc.banner @my_inst
  sc.action { sleep 2 }
end
```

The 'old' way is preserved. 
But, as before, instance variables are not available.

```ruby
@my_inst = "My Banner"
SpinningCursor.start do
  banner @my_inst     # Here @my_inst is from Parser and it's nil
  action { sleep 2 }
end
```