Support for tags resource

This is conversation starter, rather than finished code.

I would like to be able to use the gem to access the [Tags resource](http://developer.zendesk.com/documentation/rest_api/tags.html). I would like to contribute a fix for this, however I'm not sure about what I should test, etc.

I'm assuming that I wouldn't be able to run the "live" specs locally - would I need Zendesk-internal systems?