Scroll to add gateway on action link. Notify user of what is happening

Implements what winnie said: https://github.com/stellar/stellar-client/issues/1006#issuecomment-59303342

Adds scrolling and an alert.

#1006 
It doesn't work if it's in `$scope.$on('action-add-gateway', function(event, params){`

Keeping it in the current location without a `setTimeout` also doesn't work.