Handle more logical types

Hey @rogpeppe !
I tried to use some logical type like `time.Time` in the example you gave me in playground, but it failed. (https://play.golang.org/p/3XUf8IgChxN)
Maybe it is not handled yet ? :) btw really good job !

```
package main

import (
"fmt"
"log"
"time"

"github.com/heetch/avro"
)

func main() {
type Foo struct {
A int
B *string
C []string
D time.Time
}
data, dataType, err := avro.Marshal(Foo{
A: 99,
C: []string{"hello world"},
})
if err != nil {
log.Fatal(err)
}
var f Foo
if _, err := avro.Unmarshal(data, &f, dataType); err != nil {
log.Fatal(err)
}
fmt.Printf("round trip: %#v\n", f)
fmt.Printf("schema: %v\n", dataType)
}

```

output:

```
2009/11/10 23:00:00 cannot make Avro schema for Go type uint64
```
thanks in advance 👍