modifications to work from @stanlemon
modifications to work from @stanlemon

- get rid of $connect, use $pdo to track connection

- extract polymorphic constructor logic to separate methods

- update comments

frederikbosch/Aura.Sql,ThisIsPuzzle/Aura.Sql,auraphp/Aura.Sql,JLacoude/Aura.Sql,sasha-ch/Aura.Sql