Added a link to the Pro API on the Pro splash page

In response to issue https://github.com/gocardless/splash-pages/issues/289

Alignment seems to be 2px off in the vertical direction - but there weren't any utility classes in the styles that could fix such a small offset...

![screen shot 2014-10-31 at 15 42 49](https://cloud.githubusercontent.com/assets/2903904/4863453/ea1d6b40-6114-11e4-83e1-7d13e0d8fb5e.png)