Add webhook request signature verification methods

[niklv]

This method is missing in recurly for node.js package
https://recurly.com/developers/reference/webhooks/#signature-verification

[nsnider7]

This would be extremely helpful. @niklv were you able to find a workaround?

[niklv]

@nsnider7 you can implement manual check https://recurly.com/developers/reference/webhooks/#manual-check

[wlechowicz]

You can, but you shouldn't have to. Other Recurly clients support this, Node.js client does not - it should.