Print invoices with balance payments

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.

Contributors:
-------------
* Virgil Dupras (virgil.dupras@savoirfairelinux.com)