GetAdditionalHandlingFee added to PayPalStandardProvider

Without this Method-Override the HandlingFee for PayPalStandard is
always 0 and no Fee will be displayed and added to Total

![paypalpaymentfee](https://cloud.githubusercontent.com/assets/2790090/4800741/49f78968-5e29-11e4-8e0f-9902fa8b6bf1.PNG)