Converts USD formatted string to integer and vice versa

Prices are being read from a file as a formatted string
i.e. "$4.25". Converting that to an integer and multiplying by 100
will make it easier to operate on.

Conversion back to a USD formatted string is for display and output
purposes.
Merge branch 'master' of github.com:jpark3000/xkcd_knapsack

* 'master' of github.com:jpark3000/xkcd_knapsack:
  Update README.md