fixes #122

`DomainClassPropertyComparator` was moved to the scaffolding plugin in Grails >= 2.3.0, so to ensure the plugin works in all versions of Grails, I've added this class to the plugin as `grails.plugin.formfields.DomainClassPropertyComparator`.

Once the project is upgraded to Grails >= 2.3.0 this class should be removed from the plugin and all usages of it should be replaced with `org.codehaus.groovy.grails.validation.DomainClassPropertyComparator` (which is only available in Grails >= 2.3.0).