Fix inconsistencies in import formatting

Per investigation after a note from Chris Bandy on commit
1612de0.

doc.go

@@ -5,8 +5,9 @@ In most cases clients will use the database/sql package instead of
 using this package directly. For example:
 
 	import (
-		_ "github.com/lib/pq"
 		"database/sql"
+
+		_ "github.com/lib/pq"
 	)
 
 	func main() {

encode_test.go

@@ -1,12 +1,12 @@
 package pq
 
 import (
-	"github.com/lib/pq/oid"
-
 	"bytes"
 	"fmt"
 	"testing"
 	"time"
+
+	"github.com/lib/pq/oid"
 )
 
 func TestScanTimestamp(t *testing.T) {

listen_example/doc.go

@@ -18,11 +18,11 @@ mechanism to avoid polling the database while waiting for more work to arrive.
     package main
 
     import (
-        "github.com/lib/pq"
-
         "database/sql"
         "fmt"
         "time"
+
+        "github.com/lib/pq"
     )
 
     func doWork(db *sql.DB, work int64) {

oid/gen.go

@@ -5,13 +5,12 @@
 package main
 
 import (
+	"database/sql"
 	"fmt"
 	"log"
 	"os"
 	"os/exec"
 
-	"database/sql"
-
 	_ "github.com/lib/pq"
 )