Conditional GET convenience method

[davecheney]

Suggestion from dsal for a Conditional GET helper

type ReaderMaker func() (io.Reader, error)

// ConditionalGet writes the body of url to w if the server provides a 200 response.
// If the response is 304, f is called to provide an io.Reader which will provide the body.
// If f is nil, and a 304 is encounted, StatusError{304} will be returned.
func ConditionalGet(w io.Writer, url, etag string, modtime time.Time, f ReaderMaker) (int64, error)