Deprecate Dataset#set

I don't like the inconsistency between this and Model#set.
Deprecate Model.set, .update, .delete, and .destroy

These make it way to easy to require a database restore:

  album = Album[1]
  Album.destroy # instead of album.destroy

The thing is, this might not even be caught by tests, since the
album instance does get deleted from the database.
Deprecate Database#transaction :disconnect=>:retry option

The :retry_on option is more general, so users should just switch
to that.
Update testing.rdoc

Mention spec_bin task, and that Sequel does not automatically
create test databases.