Fixed a crash on tables that have more than one primary key.

[iadaz]

Now adds the PK attribute commented out (and doesn't crash), with a reminder that PetaPoco doesn't support them, for example:

    [TableName("sometable")]
    //Table has multiple primary keys which PetaPoco doesn't support: ProductId, ProductType
    //[PrimaryKey("ProductId,ProductType")]
    [ExplicitColumns]
    public partial class sometable