Add Model.restore(ids) function

Modelled on AR::Relation.destroy
Merge pull request #69 from fullcirclegroup/class-level-restore

Add Model#restore(id)
destroy! when destroying deleted? records
Add test for destroy of a deleted? object
Add description of destroy! behavior to README
Remove call to update_column deleted_at that was 'deleting' things before their time
Fix double destroy test