How to: URL signature
The URL signature is provided by the `sign` request parameter.

The HMAC-SHA256 hash is created by taking the URL path (including the leading /), the request parameters (alphabetically-sorted and concatenated with & into a string). The hash is then base64url-encoded.

Here an example in Go:
```
signKey  := "4f46feebafc4b5e988f131c4ff8b5997"
urlPath  := "/resize"
urlQuery := "file=image.jpg&height=200&type=jpeg&width=300"

h := hmac.New(sha256.New, []byte(signKey))
h.Write([]byte(urlPath))
h.Write([]byte(urlQuery))
buf := h.Sum(nil)

fmt.Println("sign=" + base64.RawURLEncoding.EncodeToString(buf))
```