add catalogue method to function example group

This brings the function example group into line with the
class/define/host example groups.  Having access to the catalog in this
context is particularly useful for testing functions that create
resources. Eg.

    it 'should work make a stooge resource' do
      should run.with_params('larry')

      larry = catalogue.resource('Stooge' 'larry')
      larry[:hates].should eq 'knock knock jokes'
    end