Converted Money#cents to be the public value

Also fixed Money#to_s
if we stick with `currency.decimal_places` and you do:

    Money.new(10).format  # => $0.10

Because we're only "padding" up to 2 positions, which is what `10` cents contains, when really we want to pad to `FACTOR + 1` positions, and then take the leftmost `currency.decimal_places` digits.

This also changed an errant `ljust` to `rjust`