Cleanup of classes export.php and invoice.php by Raffael

https://github.com/modir/simpleinvoices/commit/ef3a636d4ba192645ca8dde3c5220846b313959d
https://github.com/modir/simpleinvoices/commit/042a6eb6143a2bf6f620f43aa92f30b50a84a0f5
https://github.com/modir/simpleinvoices/commit/68e271883d89f01ebc0b8ca92924c4782c1a57cd