Fixed minor bug in AddGatewayCtrl

Check GIF below. It's self-explanatory.

![out](https://cloud.githubusercontent.com/assets/464938/4278107/f1ef28a0-3d14-11e4-9413-d30f78309c9a.gif)

https://github.com/stellar/stellar-client/commit/a8b5f37e2814a1aa4352b3103aa48badfb94a22c#diff-974c97af774f062482c7f927b280a7feL21

This was originally named ```gatewayDomain```.