Added logo image and border color customisations

PayPal offers the ability to specify a logo image and the border gradient color (as well as a header image). This pull request adds the ability to set both of these.
I was just looking into this to see the difference between HDRIMG and LOGOIMG.
Some conflicting documentation on Paypal.

Does it deal with a 190px image correctly? (I've not managed to test it yet)
https://developer.paypal.com/docs/classic/express-checkout/integration-guide/ECCustomizing/#id0863B0L805Z__id086479001Y4 suggests that it's 90x60 max?
I see that https://developer.paypal.com/docs/classic/api/merchant/SetExpressCheckout_API_Operation_NVP/ does reference 190px though.

The first link doesn't mention HDRIMG at all, as far as I can see.