Fixed NullTime returns always invalid on sqlite3.

gorp.NullTime is always return invalid if using sqlite3.

example:
```go
type Test struct {
        Id    int           `db:"id"`
        Test  gorp.NullTime `db:"test"`
        TestA time.Time     `db:"test2"`
}

_, err = dbmap.Select(&list, `select * from TEST where id=?`, id)
if err != nil {
        panic(err)
for _, v := range list {
}

// always output false
fmt.Println(v.Test.Valid)
```

I fixed this probrem with handling []byte in NullTime.Scan().  And, I added tests for NullTime to find environment probrems.

thank you!