Implement large object support

This patch implements large object support as described in
http://www.postgresql.org/docs/current/static/largeobjects.html

large_objects_test.go

@@ -0,0 +1,121 @@
+package pgx_test
+
+import (
+	"io"
+	"testing"
+
+	"github.com/jackc/pgx"
+)
+
+func TestLargeObjects(t *testing.T) {
+	t.Parallel()
+
+	conn, err := pgx.Connect(*defaultConnConfig)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tx, err := conn.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	lo, err := tx.LargeObjects()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id, err := lo.Create(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	obj, err := lo.Open(id, pgx.LargeObjectModeRead|pgx.LargeObjectModeWrite)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := obj.Write([]byte("testing"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 7 {
+		t.Errorf("Expected n to be 7, got %d", n)
+	}
+
+	pos, err := obj.Seek(1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 1 {
+		t.Errorf("Expected pos to be 1, got %d", pos)
+	}
+
+	res := make([]byte, 6)
+	n, err = obj.Read(res)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(res) != "esting" {
+		t.Errorf(`Expected res to be "esting", got %q`, res)
+	}
+	if n != 6 {
+		t.Errorf("Expected n to be 6, got %d", n)
+	}
+
+	n, err = obj.Read(res)
+	if err != io.EOF {
+		t.Error("Expected io.EOF, go nil")
+	}
+	if n != 0 {
+		t.Errorf("Expected n to be 0, got %d", n)
+	}
+
+	pos, err = obj.Tell()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 7 {
+		t.Errorf("Expected pos to be 7, got %d", pos)
+	}
+
+	err = obj.Truncate(1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	pos, err = obj.Seek(-1, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pos != 0 {
+		t.Errorf("Expected pos to be 0, got %d", pos)
+	}
+
+	res = make([]byte, 2)
+	n, err = obj.Read(res)
+	if err != io.EOF {
+		t.Errorf("Expected err to be io.EOF, got %v", err)
+	}
+	if n != 1 {
+		t.Errorf("Expected n to be 1, got %d", n)
+	}
+	if res[0] != 't' {
+		t.Errorf("Expected res[0] to be 't', got %v", res[0])
+	}
+
+	err = obj.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = lo.Unlink(id)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = lo.Open(id, pgx.LargeObjectModeRead)
+	if e, ok := err.(pgx.PgError); !ok || e.Code != "42704" {
+		t.Errorf("Expected undefined_object error (42704), got %#v", err)
+	}
+}