Formatting, help comments and new IPN complete opt

- Added new user-configurable IPN complete handling option for those merchants wishing to set an order as complete automatically and still provides for existing merchants who need the order to remain in a processing state
- Improved overall look of the plugin control panel by adding logo & link to our site
- Added helpful comments for Transaction Speed and new IPN complete option handling
- Formatted XML to standardized spacing with the other plugin files