Print invoices with balance payments

Changes the default invoice layout to add a few things, the main of which is the balance payment fields. Under the Total field, 3 new lines appear:

• Previous balance: The previous balance of the customer at the date of the

previous invoice. * Payments: The amount of payments that have been made between the date of the last invoice and the date of the printed invoice. * To Pay: The total amount owed by the customer at the date of this invoice.

Also, it changes two minor and unrelated aspects:

• Replaces "Source" by "Contract Number".
• Removes the Tax column in the invoice line table.