Sequel support

Support for the Sequel ORM.
> Looks great, although at I didn't yet grasp how exactly it works in the context of this issue.

Uniqueness validation uses `where(...)` query. So I make `Enumerize::Value` properly converted to SQL using its `#value` method. This also allows using `Enumerize::Value` objects in any Sequel queries.

> I remember that there was an infinite recursion happening somewhere but now I cannot reproduce it by tests if I remove line 76. Which is weird. Were you able to reproduce that?

I can't reproduce the infinite recursion issue. Maybe it depends on Sequel version.