Fix problems when BBDB is not present and gnorb-bbdb is not loaded.

I'm experimenting with gnorb but have BBDB 2 installed, and therefore don't want to try gnorb's BBDB integration.  I don't load gnorb-bbdb, but still have to make the following changes to run gnorb without errors.

I hope that you find the patch useful.
I removed the 'require' because it is now redundant.  gnorb-org-popup-bbdb is only called via the org-agenda-finalize-hook, whose setup is guarded by a (featurep 'gnorb-bbdb), so the require is a nop.  I'm using the fact that the user has required gnorb-bbdb in their .emacs to flag that they want to use the BBDB3 features of gnorb.