print a row with hardware name and price difference

For each item that has had its price changed, print a row with the item
name and the price difference.

For example:
`eyebolt` and `washer` have a `current_price` and a `correct_price`.
remove code cruft
add inventory.txt file

Haven’t gotten this part to work yet. Need to figure out how to read in
the file, and pass the values to Hardware.new.
add README
Merge pull request #1 from akshatpradhan/challenge_144

compute the proper costs of inventory