Corrected the item list with price and VAT
Reworked sending of invoices with e-mail by using msmtp.
Added evelopment instructions
Changed the preview of invoices to be a pdf with the text 'PREVIEW'.
Added that the verification directory tree is possible to modify by the user. Modified the data sorage format keeping backward compatibility.
Improved comments.
constified al source whee needed.
Changed the application name in the GPL header to TallyBook. Corrected some line endings.