Rewrite Accountable and Taxable roles specs
Update Accountable module to new specs
Update Accountable module specs
Update LineItem::Base specs with .augment #category
Update LineItem::Base to spec
Update LineItem::Parser to new spec
Update Receipt specs, to be clearer
Rewrite Receipt#print for readability
Add .gemspec
Update executable to use gem lib
Update README, add CONTRIBUTING and LICENSE
Bump gem version