Constraints DSL editing support

In Grails 2.0, there is a dsld file that ships with the distribution. This file provides editing support in STS for various grails dsls.

This issue is about the constraints DSL. There is some initial work in the grails.dsld file for constraints, but it is incomplete. It needs to be checked over by a grails-core developer to ensure there is nothing missed. Also, there needs to be documentation added for the various constraints.