Feature request: add support for Zendesk token auth

Hi --

We're looking to use libsaas to communicate with Zendesk via their API token auth mechanism described here:

http://developer.zendesk.com/documentation/rest_api/introduction.html#security-and-authentication

However, Zendesk expects the following basic auth string: "{email_address}/token:{api_token}".

Perhaps, opening up the Zendesk.init() signature to accept a Zendesk token value would provide the ability to authenticate either with email and password or email and token.

/thor