Several slightly advance usage of Ruby

Demonstrating these Ruby concepts:
* range
* Enumerable#inject
* "Opening" existing class
* Hiding test suite in conditional

And, replacing TABs with double-spaces.
[`__FILE__`](http://stackoverflow.com/questions/224379/what-does-file-mean-in-ruby) is the name of the current file being interpreted by ruby.

`$0` is the first argument passed into `ruby`. Say if you ran `ruby foo.rb` in a terminal; then $0 == "foo.rb")

Put those two bits of knowledge together and  this line makes sense: "Hey, only run the tests if we ran `ruby fizzbuzz.rb`!"

The reason we would do this is so when we `require` the fizzbuzz file (for instance if we want to use the `fizzbuzz` method from within a sinatra application) it doesn't run the tests then.