1.0.a

I've simply implemented compound key support feature. Please review my changes. To be honest, I can't understand why `Model::Schema::local_key_column` method uses last key column. So, I made new method `::primary_keys` for compound key feature.
And, I found that `Model::new` raise error when passed argument. `::new` method transfer arguments to `Model#intialize_new_record`, but this method cannot accept that argument. I thought that `::new` method's argument works like `ActiveRecord::new` mass-assignment. My implementation potentially has security issue, might be needed `attr_accessible` feature like ActiveRecord.
Finally, I think that `Inserter#execute` have to use `DataSet#write` like `Writer#execute`.
Please review and consider as a just reference. Thank you. :)