[FIX] Fix style errors in the invoice:

- The invoice lines wrapped text poorly for long line descriptions.
  The text is now in line with the product name.
- The margins for the header and the start of the invoice were
  pretty large. These have been decreased somewhat. Various
  other margins and min-heights have also been adjusted.
- The bank statement part, if it propagated to the following page,
  overlapped with the header of that page. The header has been
  properly separated to a separate template, so that the page and
  header are not rendered in the same space for PDF reports.
- The sum and taxes part of the invoice is forced to stay in
  one part so it doesn't split on page breaks.

Closes avoinsystems#7