Add rake to Gemfile, dropped Gemfile.lock and added a gitignore

HIt a rake snag with running rake in a chapter directory to run the tests so I did some clean up.

Also, It would be awesome to explain how to use the tests in the readme for new comers (me included).

On that note, I wanted to point out that you can't run a single file to get the output. For example, I'm on chapter 17 and I wanted to add some ruby at the end of the file instantiate the class and what not. Normally you can do that and then run the file with the ruby command in terminal to see the output, but because the tests are embeded in there you can't.

For example, If I run `ruby ex_2_basic_doc_iterator_spec.rb` I get `ex_2_basic_doc_iterator_spec.rb:19:in `<main>': undefined method `describe' for main:Object (NoMethodError)`.