Add sequel support.

I am trying to migrate some activerecord code to [sequel](http://sequel.jeremyevans.net/), and this makes
aasm work for sequel out of the box. This is not yet feature complete
as for activerecord, missing some scope related features. However this
could be a start, and to be honest, since we're not using them, this is
far good enough for us.

I have tried my best to make this aligned with activerecord and mongoid,
and I've overridden `aasm_read_state` for sequel, because there's no
`new_record?` but only `new?` in sequel, and there's no `blank?`,
either. (which I believe is provided in activesupport, which sequel
does not depend on, and I don't want to force people to depend on that.)

Thanks for considering.