Problem 17: Number to word translation ...

Well, that was interesting. I'm sure there is a better solution,
but this one works. I'd like to come back to this one and refactor
the way that's working (and maybe update it to work for bigger
numbers?).
# Please enter the commit message for your changes. Lines starting
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# On branch master
# Changes to be committed:
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#	new file:   problem16/digit_sum.rb
#	new file:   problem16/digit_sum_test.rb
#