Add experimental Sequel backend

This is a draft implementation of a [Sequel](http://sequel.rubyforge.org/) database backend.

As discussed in #8, this uses a fairly "close to the metal" library that supports a wide range of database adapters. Locking is done using `SELECT FOR UPDATE` which is not always optimal. (See [delayed_job_active_record](https://github.com/collectiveidea/delayed_job_active_record/blob/master/lib/delayed/backend/active_record.rb) for alternatives.)

Even though all tests pass this could still use some work.