Configurable JSON login

Hi,

First, I've changed the dependencies inside BuildConfig.groovy because you don't need to import 'spock' on Grails 2.3.x

Also, with this change you can use the properties:

grails.plugin.springsecurity.rest.token.json.usernamePropertyName
grails.plugin.springsecurity.rest.token.json.usernamePropertyName

To configure the JSON received to generate el token.