Vending Machine Approval Kata

This is starting code for doing approval testing for the Vending Machine Kata.

1. The starting position has a functioning test for the first feature. Re-write it to instead use Approval testing.
2. Write new tests to check the other coin types work correctly.
3. Implement the "return coins" feature. You can either do this for yourself or you can take a shortcut and merge in the branch 'return_coins'. Be sure to test the new feature - redesign your printer as needed.
4. Implement the "select product" feature. Again there is a branch "select_product" if you want to take a shortcut. Write a test for this feature too.
5. Continue adding features and tests until you have completed everything in the kata description.