Add s3util.List() for getting entries in a directory

Here is my implemention for APIs proposed in https://github.com/kr/s3/pull/7#issuecomment-20946552

Note: s3uti.Open() is already taken, so I used the name NewFile() instead.
Can you propose a better name?

```go
func NewFile(url string, c *Config) (*File, error)

func (f *File) List(n int) ([]os.FileInfo, error)
```