take care of transaction's net amounts (closes #1694)
take care of transaction's net amounts (closes #1694)

The account balance is impacted by the net amount, because Paypal's fee is
deducted at source.

sputnick-dev/weboob,sputnick-dev/weboob,RouxRC/weboob,frankrousseau/weboob,sputnick-dev/weboob,laurent-george/weboob,RouxRC/weboob,laurent-george/weboob,frankrousseau/weboob,willprice/weboob,nojhan/weboob-devel,nojhan/weboob-devel,willprice/weboob,laurent-george/weboob,frankrousseau/weboob,nojhan/weboob-devel,willprice/weboob,RouxRC/weboob