Convenience method for setting bit_fields using symbols on column_name

I've been messing around with migrating some models from DataMapper over to Sequel.  Sequel-bit_fields has been super helpful.  

My suggested change more or less adds a similar behavior from the Flag type found in dm-types.  This allows assigning bit_fields by passing symbols into the model's associated column_name/property.  Let me know what you think.
I think you can set the actual value via:

```
self.values[:status_bits]=args.first
```