Added tests for each, [], drop, caching, empty?

* Added some tests for the mentioned methods so they will serve as live examples of the code (easier to understand what they're doing)!

* Also did a couple of Refactoring on the lazy_stream.rb

* Will add the rest of the tests/Refactorings soon!

Thanks for the nice blog post and interesting example. I really love SICP concepts.
Regards,