Sequel Support

This is what I hope is the beginning of support for Sequel in Sorcery.  These changes are very bare bones, just enough to get things working on my side.  I think I made all of the correct additions.  Please review and let me know if you would like to include this and if there are any changes/improvements you would like me to make.

Details:

This change adds two dependencies.  One is for the Sequel gem itself and the other is for an external plugin called `sequel_simple_callbacks` which gives Sequel an ActiveRecord-compatible API for class level hooks (`before_create`, `after_create`, etc).

I have not added unit tests yet as the GitHub page lists the build as currently broken and that was my experience when when I tried to run the specs locally.  When the broken build is resolved on the current version, I will add unit tests to exercise the new Sequel specific code, provided this is a feature you would like to include.

Thanks.