Added OpenLDAP backup using slapcat

Summary of  #278 and #397 pull requests. Tested on debian7, works great.
If there are no observations I'll add an acceptance spec and a rake task same as for other databases.

You can close #278, @deltadd is my supervisor, this code is written under his control, future development on this feature will be from my fork.

I really want to hear @stefan-lz opinion about that pull request, I'll be glad to cooperate on that feature.