Correctly implement database/sql/driver.Driver for better wrappability

[igorwwwwwwwwwwwwwwwwwwww]

I was trying to wrap pq with instrumentedsql, but it does not implement the database/sql/driver.Driver type.

Minimal example:

package main

import (
	"database/sql"

	"github.com/lib/pq"
	"github.com/luna-duclos/instrumentedsql"
	instrumentedsqltrace "github.com/luna-duclos/instrumentedsql/opentracing"
)

func main() {
	sql.Register("instrumented-postgres", instrumentedsql.WrapDriver(pq.Driver{}, instrumentedsql.WithTracer(instrumentedsqltrace.NewTracer(false))))
}

Compile error:

./minimal.go:12:76: cannot use pq.Driver literal (type pq.Driver) as type driver.Driver in argument to instrumentedsql.WrapDriver:
	pq.Driver does not implement driver.Driver (Open method has pointer receiver)

This patch aims to address this by changing the pointer receiver to a value receiver.