Promotability,Demotability and Sequence sanitization

Reason: Upon using this gem, I was faced with a requirement of promoting and demoting the record in the sequence. And so I had to implement them in this fork.

Description: Two new methods, promote and demote have been added to the module allowing the developer to promote a model to a value 1 less than its sequence_id or demote it to a value 1 more than its sequence_id and to reorder all the sequence_ids to 1..n in ascending order as they were previously.