Make default_scope optional

I added the opportunity to make the default_scope optional. This is inspired by the commit of the fork of joshuap in the rails4 branch. (https://github.com/joshuap/paranoia/commit/ff15d315e0ec425b4e4183742485abe53cd22ef1).

I would be glad if this is going into the master branch of radar/paranoia. I hope this helps the community.