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.
Hi Tom,

I think it’s because there’s an “old” checkout style (which prominently featured the header image) and the “new” style, which you can still set a header image, but also a logo to replace the business name and the colour of the border gradient.

Here are screen-shots of the two styles:

**Old style** (with header image)

![image](https://cloud.githubusercontent.com/assets/167312/3227994/23d9cb34-f082-11e3-87f1-02bd33c1de31.png)

**New style** (logo would replace “Your order summary”)

![image](https://cloud.githubusercontent.com/assets/167312/3227997/29b90574-f082-11e3-98f4-cec3c466b4f2.png)