Show gateway url (and not federation) in trading widget

Before, it used to reverse federate at stellar.org. Now it shows the gateway url

![screen shot 2014-09-29 at 3 20 08 pm](https://cloud.githubusercontent.com/assets/5728307/4450550/e4aeb3a0-4826-11e4-8746-55049f556de2.png)

The way links are created (prepending with `http://`) is the same as what the gateway links in `manage currencies`->`trusted gateways` does.

https://github.com/stellar/stellar-client/issues/885