Add support for BRANDNAME

PayPal Express Checkout allows you to optionally send your company/brand name to customize the header (if you aren't using a header image) and the cancel link on the PayPal checkout page (see image below). This PR adds support for setting the brand name.

![pay_with_a_paypal_account-5](https://f.cloud.github.com/assets/755001/2201257/9884b978-98ec-11e3-8300-eb2a5de30db9.png)