PostgreSQL backend

[nalgeon]

First-class support for Postgres backend. For example:

connStr := "postgres://redka:redka@localhost:5432/redka?sslmode=disable"
opts := &redka.Options{DriverName: "postgres"}

db, _ := redka.Open(connStr, opts)
defer db.Close()

_ = db.Str().Set("name", "alice")
_ = db.Str().Set("age", 25)

name, _ := db.Str().Get("name")
fmt.Printf("name = %v\n", name)

age, _ := db.Str().Get("age")
fmt.Printf("age = %v\n", age)

name = alice
age = 25