Fixed major bug: "Access invoices" permission doesn't work and invoices are always visible for anyone

Fixed bug: Drupal 6 to 7 upgrade path causes an PDO exception for the column "created" in the table "invoice_items"
Fixes #929664: Ability to specify state

Node language is set to undefined (und)

1) External stylesheets are disabled by default in dompdf 6.x, so the stylesheet is now used as inline style.
2) Fixed bug: Invoice items table that spans multiple pages breaks PDF rendering

2) Removed several default drupal form fields that are not relevant for the invoice node

2) Refactored variables names

Improved user experience, the form is no longer submitted the enter key is pressed. So an invoice item can be added whith the tab and enter key.

* Refactored invoice.js
* Cancel button when editing invoice item
* Confirm message when clicking on a delete icon
* After adding an invoice item move the cursor back to the invoice item description field

* Fully implemented permission "administer own invoices"
* Fixed issue #784530: Change of the owner of an invoice do not to change the owner of the invoice items