Added setting 'remove_duplicate_transactions'

This setting controls whether or not to remove duplicate transactions.
If you download transactions from 2 accounts that include transactions between
then, you should use the default 'true' as otherwise the transactions appear
twice which results to uncorrect balances. Only the transaction with the amount
>0 is used, the other one is calculated automatically.

However, when you don't download both transactions, you need to switch this
setting to 'false' in order to have a correct balance on your account.
Added support for ING savings accounts

ING has saving accounts with the same account number as the main account :-s
Skip lines without a valid account to prevent import error in app
Fixed a bug in the amounts when you're using ING Bank export file

ING uses a special field 'Af/Bij' to specify a + or - amount.
The script is now properly handling this field so amounts should
be ok now.