Add basic attribute Type coercion

I was evaluating Redis ORM/Model libraries and noticed redis-orm did not support attribute types. Basic attribute types could be added to coerce the String values loaded from Redis back into native Ruby types. Example:

```
TYPES = {
  ...
  Date => proc { |value| Date.parse(value) if value },
  ...
}

class Homework < Redis::ORM
  attribute :description
  attribute :due_date, Date
end
```
