Field tags now allow you to declare a primary key or autoincrement field

With this patch you can declaratively say:

type Product struct {
    Id         int64     `db:"product_id" flags:"pk, autoincrement"`
    Price      int64     `db:"unit_price"`
    IgnoreMe   string    `db:"-"`
}

so that your AddTableWithName will be simpler like:
t1 := dbmap.AddTableWithName(Invoice{}, "invoice_test")

instead of using SetKeys to specify keys on fields.