Add API call to list ticket comments

Hi,

Awesome work with this wrapper, really helpful! 

I found a bit of a problem though while loading ticket comments: Using the call ```t = zendesk.list_comments(request_id=<ticket_id>)``` won't list internal (not public) notes. This small addition fixes this, and allows to load internal notes together with the public comments with the call ```zendesk.list_ticket_comments(ticket_id=<ticket_id>)```