Editing a group router forces you to define a new one

![screenshot 2014-05-27 11 12 16](https://cloud.githubusercontent.com/assets/1228977/3089489/570697b6-e57f-11e3-88da-dad60c0d60ae.png)

Checking the "Delete" box on the new one lets you continue, but that's not very optimal :grin: 
Maybe we should add a null value (as the default)? Then we can add the new entry if both group and endpoint are set, ignore it if both are unset, or return a validation error if one is set but not the other.