Add method to generate a webhook signature

[donaldjarmstrong]

This method would be used to validate the webhook signature is valid.

Based on the documentation:

The signature is calculated as follows:
SHA1-Hex-Digest("{data}{timestamp}{api_secret}")

Usage:

    var data = response.body;
    var x_ts = response.header['X-Cld-Timestamp'];
    var x_sig = response.header['X-Cld-Signature'];

    var sig = cloudinary.utils.webhook_signature(data, x_ts)

    if (sig == x_sig) {
        console.log('valid');
    } else {
        console.log('invalid');
    }