Expose 'catalogue' method as consistent API in example groups

In rspec-puppet pre-1.0, the catalogue method was available in all example groups with no arguments.  This has been restored, so the catalogue is easily accessible in derivative example groups where subject is overridden (e.g. so subject can be redefined as a resource inside the catalogue).

I don't think without this there's an easy way to get at the catalogue without knowing the parent example group type (class, define, host).