Maven addon should support getting the groupId from the parent

We are trying to figure out ways to use Roo with existing, multi-module projects. The first immediate problem we run into is the Maven addon's parsing of the POM. Specifically, when it tries to figure out the groupId, it only looks for an explicit groupId. In many cases, we don't specify one - in this case, it is inherited from the parent.

I've attached a simple patch that will - if no explicit groupId is found - look at the parent element to see if it can get it from there.