Added Zendesk Support

Hi Tambet,

I've added support for the Zendesk cloud based helpdesk. (www.zendesk.com), I've been using my fork for a couple of days and it seems to work well, but I'm far from a professional JS dev, so you may want to take a close look over the code (it's based mainly on your old github button).

If you need access to Zendesk to test with, just ping me on @GlennPegden.